Pay attention to evening owls

Abstract

Our understanding on the functions of circadian clocks has deepened at a pace in recent years. Elucidation of the mechanisms of action might pave the way to a range of interventions of use in clinical practice in many fields of medicine.

Key words::

• Disease
• disorder
• diurnal
• eveningness
• hazard

Key messages

• Annals of Medicine has contributed through its volumes to discoveries of the circadian clocks.

• There are currently available quick and easy methods for assessment of chronotype.

• Disruption of circadian clock functions has a marked impact on health.

Circadian clocks are universal and evolve their properties when subjected to selection. Natural variation in the functions of the circadian clocks can contribute to variations in fitness within specific environments, since the magnitude of a deviation of the circadian period, which is normally entrained to the 24-hour cycle on a daily basis, is inversely related to the lifespan, for example, in rodent and primate species (1). Why would then an individual have a circadian clock with a period other than 24 hours? One explanation is provided by experiments demonstrating that pacemakers with periods of slightly longer than 24 hours enhance the individual's ability to react to resetting stimuli and to track dawn in general (2) and thereby favor the acuity from season to season among individuals living at higher latitudes in particular.

However, the benefit from a period being slightly longer than 24 hours is easily lost during the late spring and early summer as well as during the late fall and winter under pressure from lengthening of the circadian period, which induces insomnia that is indeed common in summer as well as in winter (3). In addition, the deprived night-time sleep wears out this benefit. Other primates do sleep for 10 hours per day on average (4), whereas within the past 100 years humans have wanted to cut down their night-time sleep to be close to 7 hours per day on average.

A recent hypothesis claims that circadian desynchrony (when the circadian clock becomes misaligned with the daily photoperiodic cycles), which has accompanied the availability and uninterrupted use of electric lighting in the modern world, induces metabolic dysfunction predisposing to obesity (5). This hypothesis is supported by data demonstrating that there is a shortening of workday sleep together with a progressive delay in chronotype which equals the behavioral trait of preference to time the daily activities, suggesting a decreasing strength of time-givers due to less light during the day (6). This is likely to be due to spending more time working indoors and less time outdoors, and having more light pollution during the evening and at night. Moreover, it is likely that there are additional reasons for the progressive delay in chronotype at population level and the hypothesized predisposition to obesity, such as less exposure to cold during the winter, less physical activity, less restricted meals, or more sedentary activities in the evening with drinking alcohol and smoking, for instance.

If this above holds true, then why are there more morning-types among the elderly? I see that this is due to the cross-sectional study design with which data have been collected. Currently, there are no longitudinal data on the same individuals assessed with the same method, except those from a cohort study in which 190 twins were followed up for 5 years (7). These results yielded that 7% of the individuals changed from evening-types to morning-types, whereas 11% changed from morning-types to evening-types. As such, this finding disagrees with the common view according to which persons become more of morning-types with ageing. An explanation of it might be, and I call it here the diurnal owl hypothesis, that evening-types or ‘evening owls’ tend to die younger than morning-types or ‘morning larks’, but whether this is because of a drive for delays in chronotype at individual level and subsequent circadian desynchrony is not known and needs verification through experiments and replication studies.

However, this view might not be far-fetched, since thus far all the health hazards assessed so far appear to cluster and be more frequent among the evening-types, including the unhealthy dietary habits and alcohol use, smoking and nicotine dependence, and the increased risks of insomnia or its symptoms, depressive symptoms and disorders, infertility, hypertension, type 2 diabetes mellitus, and sleep apnea (8). Evening-types by definition do have greater deviations of their circadian period from the 24-hour cycle on average than others, but it is not currently known in detail how this property might link to a greater morbidity or mortality in humans.

Annals of Medicine has contributed through its volumes to discoveries of the circadian clocks by publishing review articles. By browsing the archives of the journal, readers have been able to introduce themselves to the bridging approach of molecules to behaviors (9), the effects of the night-hormone melatonin on the gate of sleep (10), the human clock genes (11), the interplay of central and peripheral clocks in cardiovascular and metabolic functions (12), the influence of the night-factor nocturnin on body fat composition (13), a range of interventions that can be targeted on circadian clocks in obesity (14), and the genetic variants of clock genes in cancer (15), for example. Along with these and many additional reviews, Annals of Medicine has also published some original articles on the topic of circadian clocks during the years, such as the use of melatonin as a circadian biomarker (16), and the genotypes of risk for winter depression (17).

The disrupted circadian clocks have a marked impact on health (18). Assessments of circadian desynchrony and the underlying chronotype should therefore be of interest not only in academic research, but also in clinical practice. Currently, there are available quick and easy methods for assessment. In addition to the screening and identification, there are nowadays effective non-pharmacological interventions as well as selective medication options that can readily be used for treatment in cases where there is circadian misalignment and subsequently in the long term the increased risk of health hazards. So, I think that not only the physical activity, dietary intake, and sleep are health imperatives, but so are also our preference and timing of the daily activities.

Declaration of interest: The author reports no conflicts of interest. The author alone is responsible for the content and writing of the paper.