35 years of light treatment for mental disorders in the Netherlands

Abstract

Background

Light therapy (LT) for Seasonal Affective Disorders (SAD) has been a well-known and effective treatment for 40 years. The psychiatric university clinic of Groningen, the Netherlands was an early adopter and started research and treatment of SAD in 1987. Research projects on mechanisms, the role of the circadian system, treatment optimization, and investigating new areas for the effects of light treatment have been carried out ever since, leading to a widespread interest across the country.

Objective

To provide an overview and description of the historical development of LT for mental disorders in the Netherlands.

Methods

A non-systematic, review of research on light treatment for mental problems in the Netherlands, published since 1987 was conducted.

Results

The fields of LT and chronotherapy are strongly based in the scientific interests of both chrono-biologists and therapists in the Netherlands. LT has shown effectiveness in treating mood disorders. Likewise, results for other mental disorders have shown some promise, but so far, the outcomes are not always unequivocal and have not always been based on robust data. Ongoing research is discussed.

Conclusions

LT, and in addition exposure to the right light at the right time is an important issue in mental health. Over the past 3 decades research on light and LT in the Netherlands has become well established and is still growing.

Keywords:

• Light treatment
• mood disorders
• mental disorder
• chronotherapy
• review

Introduction

Light treatment (LT) for mental disorders is a relatively new intervention which first became popular about 35–40 years ago in a small number of clinics in the US and Europe. Although historically a small number of descriptions of the positive effect of light on mental health were reported (for a review see [1]), the now classic study by Rosenthal et al. in 1984, describing the symptoms of seasonal affective disorder (SAD) and the positive effects of light treatment [2], was the starting point of numerous studies on this subject. It also led to the introduction of light treatment for mental problems in clinical practice. For this type of LT, patients are sitting in front of a light box with their eyes exposed to electric light.

A few years later, in 1987, professor Van den Hoofdakker at the University Clinic of Groningen, The Netherlands, initiated a research program using light treatment [3]. In this contribution an overview of the developments following the introduction of light treatment in clinical practice in the Netherlands is provided, primarily based on studies of the University of Groningen. Nowadays, light treatment is administered to patients with a variety of psychiatric diagnoses, however, in the early days it was mainly used for the treatment of SAD. In this contribution, an overview is presented of the clinical studies examining light treatment in patients with mental problems in the Netherlands. Also, some related aspects as a Dutch collaboration network of clinicians using LT in different hospitals and the importance of light in the architecture of hospitals is mentioned, followed with some new developments.

Seasonal affective disorder

Seasonal affective disorder (SAD), winter type, is a form of recurrent depression, typically occurring annually, with the onset of symptoms in autumn/winter followed by a spontaneous recovery in spring/summer. Currently it is mentioned as a specifier of Major Depressive Disorder or Bipolar Disorder in the DSM-5 classification system [4].

Timing and duration

After the publication of the study by Rosenthal et al. [2], discussions arose regarding the working mechanism of light treatment and the etiology of SAD. One of the most influential hypotheses was the phase-shift hypothesis by Lewy et al. [5], which stated that SAD patients show an abnormally early or late phase of the endogenous circadian clock, resulting in an aberrant phase relation with their sleep-wake cycle. The majority of SAD sufferers were thought to be phase-delayed, a minority phase-advanced.

On the basis of this hypothesis, it was postulated that the correct timing of light treatment was essential for its therapeutic success and that the majority of SAD sufferers would therefore benefit from light exposure in the morning. Investigating this topic was the kick-off of a long-term interest in LT for mental health in the group of professor van den Hoofdakker.

In a small study comparing the effects on SAD patients of light treatment in the morning versus treatment in the evening, no significant differences between conditions were found [6]. Subjects were offered 3 h of light with an intensity of 2500 lux on 3 consecutive days. In both groups (one with light treatment between 9 and 12 am, response rate 57% and one with light treatment from 6 till 9 pm, response rate 50%), participants showed significant improvements in depression and fatigue.

After the introduction of light with an intensity of 10,000 lux, the duration of the sessions was reduced from 3 h to 30–45 min [7]. In a study using this higher-intensity light for 30 min a day, no significant differences were found between the effects of light offered over 4 d in the morning, afternoon, evening or over a combination of 2 d of morning light followed by 2 d of evening light or vice versa. Response rates ranged from 50% (combination of evening and morning light) to 80% (evening light), with no significant differences between them [8].

While some studies have found that morning light was superior [9], others have confirmed our results in showing no significant differences in effects between morning and evening light [10]. We are not aware of any study showing the superiority of evening light. For this reason, also taking into account the chronobiological hypothesis, and for pragmatic and practical considerations, it has become common use to offer light treatment for SAD in the morning.

While the duration of light treatment sessions per day was reduced, the number of days required to reach an effect has been a topic of debate until the present day. The effects of 2 weeks of LT were reported to be superior to those of 1 week [11], but it remained unclear whether the additional week of exposure in the second week was actually needed to reach this effect. Meesters described that the effects of LT assessed 1 week after completion were superior to the immediate results after the 1-week treatment [12]. This indicates that compared to the week following LT, either the method of using a retrospective assessment results in improvement one week later or that indeed, the improvement in the week after LT continues for some time. Expectations may also play a role as it has been suggested that prior knowledge of duration of treatment, does, in fact, affect response to treatment. Levitt and Levitan [13] showed that short durations of LT (2 weeks) can be equally effective as longer durations (5 weeks). These results suggest that responses are faster in the group receiving LT of shorter duration LT. A retrospective study comparing therapeutic outcome between studies with 1 vs 2 weeks of LT reached the same conclusion [14].

Prevention

By definition, SAD winter-type is a recurrent disease. Efforts have been made to find treatment options to preventing full-blown depressive episodes. Research has shown that LT offered at the first signs of a depressive episode can prevent the development of a full-blown depressive episode in the remaining part of the season [15]. In an attempt to replicate the study, it was shown that LT offered near the onset of a depressive episode could delay the occurrence of the next episode [16]. Offering LT at the beginning of autumn, before any depressive symptoms were present, did not prevent a depressive episode in the following winter period [17].

Starting LT before SAD symptoms are present, and continuing this treatment into the period in which patients developed complaints in previous winters may lead to the early prevention or treatment of a depressive episode [18]. Similarly, the use of light visors (wearable light fixtures on the head) during the winter has made it possible to prevent a depressive episode [19]. This latter study is the only one to meet the criteria of a Cochrane review on the potential of LT in preventing the onset of depressive episodes in SAD patients [20]. However, due to the small sample size and some methodological limitations, the evidence that LT can be used as a preventive method for winter depression, is inconclusive and further research is needed to find more support for this treatment modality.

Blue light

The discovery in 2000 of a new photoreceptor, the retinal ganglion cells containing melanopsin, has had a significant influence on the field of SAD research and light treatment [21–23], also in the Netherlands. These non-image forming (NIF) photoreceptors are the gateway to the brain area generating circadian rhythms and are most sensitive to light with a wavelength of 470–490 nm (blue light) [22,23]. Based on these findings, numerous studies have been conducted studying the role of these melanopsin cells in LT effects in SAD. Blue-enriched light fixtures have become available for therapeutic purposes. In a study comparing the therapeutic outcome of blue-enriched light (17,000 Kelvin (K)) in 20- or 30-minute sessions with that of standard bright light, no differences were found in the effectiveness of SAD treatment. Responses were high in all conditions [24].

To investigate possible saturation effects, this study was repeated with low-intensity (750 lux) blue-enriched light and compared to bright white light. Here no differences in treatment outcome were found either [25]. This led to the conclusion that either the success of standard bright light is already so high that further improvement by changing the light spectrum is impossible, or that melanopsin cells do not play a major role in the effectiveness of LT in depression treatment.

After blue light-emitting diodes (LEDs) were invented, narrow-band light sources matching the maximum sensitivity of the NIF photoreceptors became available and were used in various studies. Exposure to blue light was found to be superior to exposure to red light [26,27]. The effects of blue-enriched LED light were also superior to placebo when treating SAD [28]. Also, no differences were found between exposure to narrow-band low-intensity blue light and standard bright white-light in the treatment of SAD [29] and of sub-SAD (winter-blues) [30]. A study comparing blue light (465 nm) with blue-free light (595–612 nm) found no difference in treatment outcome [31].

A recent review article concluded that the efficacy of blue-light therapy remains unproven in the treatment of seasonal and non-seasonal depression. The authors recommend that more studies with larger sample sizes and longer duration be conducted [32].

Placebo?

Initially a rather skeptical attitude towards the effectiveness of light treatment was quite common. When recruiting participants at the start of a research program, with a description of the symptoms of SAD provided, all GP’s in the northern region of the Netherlands were asked to invite their patients to participate in the study. This yielded no response at all. The first participants were only included in the program after publicity on a regional radio station and in some popular magazines.

A challenge in clinical studies on the effects of ocular LT is that creating a methodologically sound placebo condition is impossible (i.e. a condition without participant noticing the supposed effective part of LT). Several attempts to investigate a sort of placebo treatment have been investigated.

In one of the initial studies the effects of regular light treatment were compared to those of imaginary light treatment as a kind of placebo condition (participants got the suggestion of light by a therapist in a dark room) and no differences in response were found immediately after the treatment of patients with SAD had finished. Both groups showed satisfactory levels of recovery. However, the group receiving imaginary light treatment relapsed after a week, unlike the group who were offered real light. So there appeared to be a difference in effect after exposure to the suggestion of light compared to real light, suggesting a real, additional effect of LT beyond the placebo effect exists [33].

In 1998, Campbell and Murphy published a study showing positive results in influencing the human circadian clock through extra-ocular light exposure (light applied to the skin behind the knees) [34]. If it were also possible to achieve similar non-image forming effects with LT for depressive symptoms, it would create an opportunity to investigate the effects of extraocular light in SAD sufferers and compare it to a methodically sound placebo condition. Although this study was methodologically sound, it failed to show differences in treatment effects between LT via the skin and placebo [35]. Actually, various other studies trying to replicate the results of Campbell and Murphy were also unsuccessful [36,37], and the current consensus is that non-image forming effects of light in the visible spectrum can only be obtained via the retina and not the skin.

In light research, various placebo-like conditions have been used, such as extra-ocular light [35,38], invisible light (looking at light bulbs (black painted) without visible light [39], imaginary light [33], low-intensity light [40,41], or other placebo conditions totally unrelated to light, like a deactivated ion generator [28]. Responses to these ‘placebo’ conditions varied from 36% to 46%, which is generally lower than responses to standard LT which range from 50% to 80%. In a recent meta-analysis of randomized controlled trials comparing LT to a placebo condition (dim light or sham/low density ion-generator) the LT condition was superior [42].

Theoretically, it is still possible that the therapeutic effects of LT for depression can be explained by a specific therapeutic variable unrelated to light, such as improvement in day-night structure, recognition of complaints by professionals, or placebo. These types of variables definitely play a role, but it would seem unlikely that the quick and robust effects of LT can have existed for decades without having any intrinsic effects.

It is also unlikely that a single factor or explanation can account for all therapeutic effects of LT. If appropriately timed, LT can influence the biological clock (resynchronize it), and may affect sleep pressure and alertness, and the like [43–45]. The effects of LT on mood depend on light intensity, wavelength, duration, time of day and individual circadian rhythm and more [46]. Not every aspect carries the same influence or is even necessary for therapeutic outcome. Knapen et al. [47] for example, found no relationship between chronotype and therapy outcome with LT at a fixed timepoint, indicating that the anti-depressive effect of morning light in SAD patients is an unlikely explanation of a phase shift of the biological clock. A similar conclusion was drawn from a meticulous force desynchrony study comparing circadian clock characteristics prior to and post LT in SAD patients in winter [48], and also from another study in Switzerland showing positive effects without phase shifts and phase shifts without positive effects [10]. Recent insights into the connection between melanopsin-containing retinal ganglion cells with areas involved in mood regulation (lateral habenula) suggest there may be a different pathway for the therapeutic effects of light therapy other than phase shifts of the circadian system [49,50].

In the studies mentioned above, only adult participants were included. In some case studies successful LT in seasonally depressed children has been shown [51,52]. LT might be a promising treatment for adolescents (mainly between 12 and 18 yr.), but so far there has been a lack of clinical trials to substantiate the evidence [53].

Non-seasonal depression

In addition to the positive results of light therapy in SAD, positive effects in non-seasonal disorders have been reported as well [54,55]. Also LT as augmentation to standard pharmacological treatment for depression is described as successful [56]. In the Netherlands, the use of LT in special groups of non-seasonal depressions is investigated in the studies mentioned below.

Elderly people

A study of non-seasonal depressed patients aged 60 years or older, showed that the effects of 3 weeks of LT were superior to those of a placebo condition. Mood and sleep efficiency improved [41].

Since cognitive decline, sleep, mood disorders and similar factors impact the care and life of elderly patients with dementia, possibly disrupting the biological clock, the effects of brighter light in their living rooms were investigated. In a group of demented elderly individuals (mean age 85,5 yrs.) in various care homes the effects of long-term daily exposure to bright light (about 1000 lux) in living rooms was compared to those of subjects living in living rooms with dim light (about 300 lux) during the daytime. The brighter light condition showed a moderate improvement of some cognitive and non-cognitive symptoms, including mood and behavior and attenuated cognitive deterioration [57].

Review articles on light therapy studies in older individuals have positive effects of LT on geriatric non-seasonal depression [58,59], especially improving sleep patterns [60,61]. Its effects on agitation and depression are less clear [61]. The majority of review articles conclude that sample sizes in most studies are too small to draw definitive conclusions. Furthermore, the manner of exposing people to light, the nature of the equipment, as well as the duration and timing of light differ and are sometimes problematic [60].

A study with SAD participants shows that therapeutic effects in subjects staying for 6 h a day in a brightly lit room were similar to the use of traditional light boxes [62]. These findings can make the use of light in geriatric care homes more feasible and more easily applicable.

Adjunctive LT on an acute psychiatric ward was feasible, well-tolerated with very few side- effects and also reduced symptoms of depression [63]. More in general, an updated meta-analysis suggests that light treatment has mild to moderate effects in reducing non-seasonal depressive symptoms and can be used as a treatment for depressive disorders. The authors acknowledge the limitations of the evidence due to the small sample sizes of the studies included [54]. Although more systematic reviews/meta-analyses conclude that LT has positive effects in reducing non-seasonal depression symptoms, there is still some criticism regarding the quality of these studies [55]. Therefore, the evidence should be interpreted with caution, and further investigations with larger sample sizes are needed.

Pregnancy

Approximately 11-13% of pregnant woman suffer from depression. Compared to antidepressant drugs, LT is a safer treatment option with minimal side effects for mother and child. Depressed pregnant women treated with 5 weeks of bright LT showed greater improvement in depression compared to the effects of placebo dim red light [64].

However, in a Dutch study comparing the effects of bright LT (9000 lux, 5000 K) with those of dim red LT (100 lux, 2700 K.) in depressed pregnant women (12–32 weeks gestational age), no differences in therapeutic responses were found. Both groups showed significant improvement [40]. It remains unclear, however, why there was no significant difference between the active treatment and control group.

In recent studies on women with perinatal depression (pregnant women or women in the first year postpartum), bright LT was found superior to placebo [65,66]. LT seems, therefore, to be a promising treatment modality for depression during pregnancy, but again, further studies with larger sample sizes are essential for more robust conclusions. Another recent study reports that a combination of 1 night of critically-timed wake therapy followed by LT was very effective, and well-tolerated in treating peripartum depressed patients [67].

Premenstrual syndrome/premenstrual dysphoric disorder (PMS/PMDD)

To investigate the possibility of acute changes in cognitive empathy and their impact on mood, a single light-therapy session (blue-enriched polychromatic light, 5000 lux, 17000 K) was offered to women suffering from PMS/PMDD and compared to the effects of a sham condition with the same light but at an intensity of 200 lux. No differences in cognitive empathy were found between the two groups, but as little as 30 min of LT led to mood improvements in the women not using hormonal contraceptives though not in women using hormonal contraceptives [68].

In a review article about the therapeutic options to treat PMS/PMDD, the use of selective serotonin reuptake inhibitor antidepressants is recommended. LT is described as a promising option, but it lacks sufficient evidence [69].

In another recent study, 7 d of morning light treatment following a night with advanced/restricted sleep proved effective in treating PMDD. This is described as a safe, efficacious, rapid-acting, well-tolerated, non-pharmacological, non-hormonal, affordable, and easily repeated home intervention for PMDD [70].

Bipolar disorder

The majority of participants in SAD studies suffer from Major Depressive Disorder (MDD) during the winter. In an open trial with winter-depressed patients suffering from bipolar disorders, exposure to light treatment (10,000 lux between 8:00 am and 11:00 am) for 7–21 d showed beneficial effects [71]. This research followed the guidelines of the recently published Dutch protocol for light therapy in bipolar disorder, which also recommends using light treatment for depressive episodes in bipolar disorders without seasonal pattern [72].

A side effect, in 2.99% of the subjects in the van Hout et al. study [71] was a hypomanic state; with no observed instances of manic decompensation after light treatment. There has been some discussion in the field about the proper timing of light treatment for bipolar disorders.

Because of the potential risks of (hypo)manic decompensation and midday effects of exposure to light treatment, this timing was recommended in a study on the positive effects of light treatment at midday [73]. This suggestion was considered rather controversial in the field and differs from the results from other studies. Practice in various clinics was the administration of light treatment in the morning with beneficial effects and a moderate risk of decompensation [74]. More studies are needed to draw final conclusions about the optimal timing of light treatment in bipolar disorders [75]. From a review and meta-analysis study on the use of light treatment in bipolar disorders some (non-conclusive) evidence was found for the positive effects of adjunctive light therapy in the treatment of bipolar patients, but the authors also concluded that there was no risk of an affective switch beyond the normal risk when treating bipolar disorder patients [76]. Other recent reviews and meta-analyses have also reported positive effects of LT as (adjunctive) treatment in bipolar disorders [77–79]. However, the number of studies included in these analyses is rather small, so more studies with larger populations are needed.

Attention deficit hyperactivity disorder (ADHD)

The neuropsychiatric disorder ADHD is characterized by symptoms of inattention and/or hyperactivity. Sleep disturbances are highly prevalent both in children and adults, with Delayed Circadian Rhythm Sleep-Wake disorder being the most common. In a study of 40 adults with ADHD, 78% suffered from sleep onset insomnia (SOI). In this group, dim light melatonin onset (DLMO), which is a marker of the endogenous biological clock, occurred 1.3 h later compared to DLMO in the non-SOI group, and as much as 1.8 h later than in an age and sex-matched control group [80].

In an attempt to phase-advance the circadian rhythms of adult ADHD patients and investigate the effects on ADHD symptoms, Van Andel et al. [81] performed a three-armed randomized clinical chronotherapy trial (RCT) on sleep education in combination with appropriately timed (1) 0.5 mg placebo, (2) 0.5 mg melatonin, and (3) 0.5 mg melatonin plus 30 min of 10,000 lux bright light in the morning. Melatonin treatment alone or combined with the LT phase-advanced the circadian rhythm when compared to placebo. No significant effects on ADHD symptoms were observed, although melatonin treatment showed a trend towards improvement. The addition of LT in the morning did not enhance the therapeutic effect, but this may be due to the strict waking up instructions given to this group or because the ADHD symptoms are not solely the result of the late clock-phase. The discussion whether ADHD symptoms are rather result from the late clock-phase, or are its consequence, is not really relevant when working with chronotherapy. This first RCT shows that appropriately timed melatonin and light can successfully phase-advance the biological clock, which may support better sleep and well-being in the end.

Other potential target groups

Some small studies and case studies have been published describing the effects of light treatment in other pathologies.

Obsessive-compulsive disorder (OCD)

In a case-study, a patient with SAD and OCD was treated with light therapy resulting in a reduction of symptoms for both conditions [82].

Two other case-studies about light treatment for seasonal OCD have also been published. One case study of a depressed patient with seasonal OCD showed a similar result after light treatment, where continued long-term antidepressant medication led to full remission of the OCD symptoms with no relapse in the next period [83].

In a third case study, a patient suffering from seasonal OCD recovered after a combination of light treatment, exposure and response prevention therapy, and pharmacotherapy and was able to discontinue the use of medication. There was no relapse in the next 16 months [84].

Burnout

Although burnout is not a medical condition in DSM5 or ICD-11 classification systems, it is included in the ICD-11 as an occupational phenomenon. It is recognized as a serious health issue and defined as ‘Burn-out is a syndrome conceptualized as resulting from workplace stress that has not been successfully managed’ [85].

Burnout is a wide spread phenomenon; in the Netherlands in 2020, 16% of employees reported burnout complaints [86].

One of the core symptoms of burnout is emotional exhaustion, i.e. a lack of energy to perform well at work. Since light treatment has shown improvement in SAD sufferers, a small pilot study was designed to compare the therapeutic outcomes of burnout patients exposed to light treatment with those on a waiting list. Energy levels were seen to improve significantly on 2 out of 3 self-rating instruments used [87].

However, light treatment with a special light-fixture in combination with Pulsed Electro Magnetic Fields and coaching had no additional effect on outcome measures compared to coaching alone in patients with work-related stress [88].

In another small study, exposure to chronotype-based bright light therapy for in-patients with severe burnout complaints effectively improved burnout symptoms and sleep problems [89]. These results warrant further investigation with larger sample sizes.

Schizophrenia

After some preliminary positive results in treating negative symptoms in schizophrenic patients by means of light treatment became known [90], a pilot study was initiated on a closed ward with severely ill patients suffering from schizophrenia. However, Roopram et al. found no beneficial effects at all after treating these patients with bright light [91].

Parkinson’s disease

A common secondary symptom of Parkinson’s disease (a disorder characterized by loss of dopamine in the nigro-striatal dopamine system) is depression. In a small trial, bright-light treatment was beneficial in treating depression and other symptoms related to Parkinson’s Disease. The effects of LT were found to be superior to those of dimmed red-light treatment [92]. In a larger RCT, the effects of bright-light treatment (10,000 lux) were compared to those of low-intensity light (200 lux) in the treatment of depressed Parkinson’s patients. No significant differences between the two conditions were found, but an improvement in depression was reported after both conditions. Bright light was found to be superior in improving subjective sleep quality compared to low-intensity light [93]. Possibly the intensity of low-intensity light was already sufficient to induce beneficial effects and made it less effective as the optimal placebo condition.

In a recent meta-analysis (based on the data of 5 studies, n = 173) beneficial effects of bright-light treatment in in patients suffering from Parkinson’s disease were reported, but again, no significant differences in effects were observed between the active intervention compared to the control light conditions (low-intensity or red light) [94]. Another meta-analysis found no significant positive effects of LT due to the heterogeneity in study protocols, small sample sizes and timing, dose and duration of LT. The authors concluded that more RCT’s with standardized protocols and larger sample sizes are needed [95].

Chronotherapy

Like LT, sleep deprivation (wake therapy), dark therapy and interpersonal social rhythm therapy are treatments capable of influencing circadian rhythms and improving mood [96]. There is some evidence suggesting that combined chronotherapeutic treatments may be more effective in treating mood disorders, but the number of studies is too small to draw firm conclusions [97].

A combination of sleep-deprivation and LT has shown some success in treating therapy-resistant depressed inpatients [98].

Hospital architecture

Artificial light from light-boxes to treat people with mood disorders in hospital are well-known, but environmental light in everyday life is equally important. In fact, it is the light entering the eye that is the important factor; it is not important where that light is coming from as long as it has the right intensity, spectral composition, timing, and can reach the eye. For example: going for an outdoor walk for a week in the morning has been found beneficial in treating SAD [99]. Walking outdoors in the morning may not be the preferred activity of severely depressed, hospitalized patients, though. But getting light through the windows of their patient rooms is still important. Several studies found that when hospitalized depressed patients or patients with bipolar disorder stayed in rooms situated to the east, the length of their stay was reduced by up to as much as 50% compared to staying in rooms situated to the west [100–103].

With the findings of these studies in mind and the clinic’s interests in a light and healing environment [104], it was decided that in the new building of the University Center for Psychiatry in Groningen, all patient rooms will be situated on the east (https://www.atelierpro.nl/nl/projects/261/universitair-centrum-psychiatrie-umcg-groningen).

In addition to the building’s architecture and windows, the way patient rooms are lit is important. Using dynamic lighting (with regulated dawn and dusk and no blue light at night) in the bedrooms of hospitalized patients with major depressions improved sleep patterns compared to the use of standard bedroom lighting [105]. The use of LED technology in the concept of dynamic lighting in bedrooms of depressed inpatients is now under investigation [106,107].

Some Dutch specialties

Chronotherapy Network Netherlands

An informal chronotherapy platform, Chronotherapy Network Netherlands (CNN)) was founded in 2013. The purpose of this platform is to serve as a meeting place for professionals and share knowledge, enthusiasm, and work experiences from clinical practice in LT and more in general chronotherapy. Chronotherapy is defined as any treatment using knowledge of 24-h rhythmicity in humans to achieve clinical effects in patients and improve well-being or health. This definition is very broad and includes nearly everything related to the 24-h rhythmicity in humans, but in this net-work LT is the main intervention that has been used. To keep things practical at the start of the network, the first invitations were limited to people using chronotherapy in psychiatry, neurology, sleep disorders, gerontology, or for the treatment of tiredness in somatic diseases. Since 2013 approximately 100 participants have shared their knowledge and experiences in this field during annual meetings, resulting in more research projects on the effects of LT in other disorders than SAD, and in spreading the practical use of LT [108].

Light café

Light therapy is usually administered in a clinic under the guidance of experts or by patients using light fixtures at home, mostly unguided. A recent initiative of the Institute for Mental Health Care Eindhoven is the Light Café. It is open to anyone eligible for light therapy. It is a pleasant, inspiring place where people can start the day with the right amount of light. While sitting in front of the light fixture, they can enjoy a cup of coffee or tea, read a newspaper, chat, or check their email. Experts are present to guide people during therapy sessions. They will also discuss lifestyle since a healthy lifestyle (a fixed daily rhythm, healthy food, exercise and social contacts) has a positive influence on mood. Research about this combination of light therapy and life style is ongoing (https://sites.ggze.nl/articles/2313), hopefully leading to scientific publications in the next future.

New developments

A new development in the field of light therapy in the treatment for well-being, sleep and mood is photo-biomodulation (PBM). This involves exposing the skin to near-infrared wavelengths. As many activities in modern society are indoors, with no direct exposure to sunlight, people are often deprived of near-infrared and other wavelengths. Relatively high levels of PBM (corresponding to the quantities in winter sunlight compared to placebo have shown positive effects on mood, the immune system, and drowsiness in healthy individuals with mild sleep complaints, but only in wintertime [109]. Although more research is needed, it is becoming evident that exposure to adequate levels of sunlight, including near-infrared wavelengths, can improve health and well-being and should earn the full attention of (interior) architects and policymakers.

In conclusion

After more than 3 decades after the introduction of LT in the clinic, LT has become an accepted non-pharmacological treatment for a growing number of disorders. LT is not exclusively limited to seasonal complaints or mental disorders anymore. The evidence of treatment outcome for other mental disorders than SAD is not overwhelming due to small sample sizes and which should encourage further research.

LT is also used successfully in treating some sleep disorders [110], but that was beyond the scope of this article.

In clinical practice LT is not always used as monotherapy. LT in combination with medication became common practice, especially in treating non-seasonal depression. In the third and last inventory of the use of LT in mental hospitals in the Netherlands in 2005, most hospitals offered LT for SAD as monotherapy, but for non-seasonal depressions most clinics offered LT in combination with other treatments. The number of hospitals that used LT was tripled (from 24 to 78) compared to a decade ago [111]. In German speaking countries more or less the same pattern was observed. Compared to a former investigation a shift from mono-therapy to a combination of LT with medication was observed in 2012 [112].

Nowadays, as awareness of the environmental impact of selective serotonin reuptake inhibitors (SSRI’s) on aquatic ecosystems is growing [113,114] more attention is being paid to non-pharmacological treatments. LT provides a good opportunity and possible alternative for medication. In the fight against global warming it is important that the energy required for electric light is generated in a sustainable manner [115].

Light has strong direct effects on mood, cognition, alertness, performance, and sleep. LT is now an accepted treatment for SAD and other depressions, and circadian sleep disorders and is mostly based on artificial light.

The most powerful source of light is the sun. Exposure to ‘natural’ daylight should be implemented in our daily lives and architects should bear this in mind when designing houses/buildings. It has also been suggested that exposure to ‘natural’ daylight is superior to artificial LT [116].

Authors contributions

The first draft of the manuscript was written by Y. Meesters and all authors commented on previous versions of the manuscript. All authors read and approved the final manuscript.

Acknowledgement

The authors are grateful to Josie Borger for the improvement of the English.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Data availability statement

No data available

Additional information

Funding

The authors reported there is no funding associated with the work featured in this article.