Introduction: special issue on “Rhythms, Calendar and Biological Processes”

All organisms use time-of-day and time-of-year information to organize their biological activities at the most profitable within each day and within each year. Natural selection has acted upon such that these timed events are repeated almost at precise intervals even when organism is confined in isolation from daily and seasonal changes in the illumination, temperature, humidity, food availability, and any other regular periodic changes in the environment. Indeed thus, organisms have been evolved with self-sustained internal timers (=clocks), which as synchronized with the periodic changes in the natural environment keep on ticking and telling organisms when the optimal time-of-day and/or the time-of-year is imminent. Unquestionably, therefore, it remains important why and how species (and individuals) are periodically active or at rest, sleep or awake, feed, and avoid feeding at an appropriate time during the day, and reproduce and ensure survival of their offspring at the appropriate time of the year, i.e. when “resources” are in plenty. Or, why a species migrates to another place located at several 1000 km when conditions become harsh and “resources” become scarce at the breeding site. Each species and individual therefore defines its relationship with the environment it inhabits, with interesting variations within and between habitats and latitudes. This is also true of several body functions that undergo daily changes, e.g. the maintenance of homoeostasis as exemplified by the core body temperature and several other functions in humans. Any mismatch with the environment is detrimental to health, and ultimately, in an extreme case, survival of the individual/species.

The appreciation of the role of clocks has gained in almost all walks of life viz. metabolism, diseases, sleep, and psychiatric disorders. Equally, the concept of biological timing has proven to be useful in the understanding of how organisms adapt and profitably live in their environment, which is essentially cyclic (periodic), and how the mistiming of daily and seasonal activities in nature can dearly cost on the health with consequences in long run on the survival. Mistiming of the food availability (or less food at the time when it is needed most, as possibly in the case of global warming) can compromise reproductive potential, as evidenced in higher vertebrates, clearly indicating the hazards for eating at the wrong time of day. No wonder eating outlets are open 24×7 in modern times.

Hierarchical systems, like the circadian clock system, often offer multiple opportunities for control – at the molecular level in the cell, synchronization of periphery from central timing system, or at the mechanism of central circadian pacing. However, the layers of regulators often heighten the chances for dysfunction too. Intensive research in the last few decades churned out barrage of discoveries associating several chronic disorders and life style related diseases to the disruption of the circadian rhythm or the clock-network. The ever-growing list of diseases includes obesity, diabetes, depression, and cancer. For example, the connection between circadian clock genes and obesity were discovered serendipitously using genetically modified mice (Turek et al. 2005). This has been extended to genome association studies prompting a clear indication of the link between disruption of peripheral circadian system and obesity (Deng et al. 2002; Kohsaka et al. 2007). Rotating night shift work disrupts circadian rhythms and has been associated with obesity, metabolic syndrome, and glucose dysregulation. Furthermore, a landmark study (Kivimäki et al. 2011) examining the association between rotating shift work (≥3 nights/month plus days and evenings) and type-2 diabetes (T2D) among 177,000 female nurses aged 25–67 at baseline followed up for up to two decades revealed a graded association between the duration of working life the nurses had been engaged in shift work and risk of developing type-2 Diabetes. The evidence from both animals and humans suggest that circadian rhythm disruption increases cancer risk. Apart from association of circadian rhythm to diseases, the timing and duration of therapy, also known as Chronotherapy, is crucial in determining the efficacy of the drugs as different types of chemotherapy drugs kill cells at different stages of the cell cycle. Several animal studies have also shown the possible impact of the loss of night on brain health and functions in an emerging ecosystem in which other diurnal species including humans may be inadvertently exposed to an illuminated night such as in an overly lighted metropolitan urban habitat (Taufique and Kumar 2016).

Chronobiology: Indian perspective

With growing evidence of chronobiology encompassing medical and other research fields including neurosciences, endocrinology, geriatrics, sports medicine, and occupational medicine, the challenge to understand the processes of “biological time-keeping” machinery or circadian rhythm remains enormous. The developed world, especially the labs in USA and Europe has progressed at a massive pace in the last about two decades in understanding the complex processes involved in all areas of chronobiology including the mammalian and human systems. However, Indian chronobiology has not progressed as much it could and should have, perhaps because of the lack of thrust and awareness in chronobiology and its clinical implications. Though many chronobiological centers have come up in India, the interest still remains mostly academic. In view of the burgeoning population in India and the population at risk shifting from 40+ to maybe 30+, India may become future global center of the lifestyle disorders including diabetes, cardiovascular diseases (CVD) with obesity as risk factor, depression and cancer. As discussed above, basic chronobiology related mechanisms sits at the root of all those diseases. Hence, Indian Society for Chronobiology has been consistently trying its best to increase chronobiological research, and has been organizing various academic and outreach activities, time to time. A set of mini-review from different Indian groups in this special volume is also an effort in the same direction. Various chapters included in this special volume are: (i) To be or not to be rhythmic? A review of studies on organisms inhabiting constant environments. (ii) Going beyond the limits: Effect of clock disruption on human health. (iii) Light spectrum and intensity, and the timekeeping in birds. (iv) Sleep in birds: Lying on the continuum of activity and rest. (v) Circadian basis of seasonal timing in higher vertebrates. (vi) Control of annual gonadal cycles in Indian songbirds. (vii) Melatonin rhythms in the pineal and non-pineal tissues and their physiological implications in sub-tropical fish. (viii) Conquering the night: Understanding nocturnal migration in birds. (ix) Insect clocks: Implication in an effective pest management. (x) Effect of urbanization on daily behavior and seasonal functions in vertebrates. This effort, I hope will serve as a catalyst to promote both fundamental and clinical research, as well as also spread the much needed awareness in this field of research, particularly among Indian researchers, and might encourage chronobiologists abroad to forge collaborative research with Indian PIs.

At the end, I profusely thank Prof. W. J. Rietveld (Editor, Biological Rhythm Research) and Prof. James W. Waterhouse (the designated Editor for this special issue) for readily agreeing to bring out this collection of mini-reviews from Indian Chronobiology community as a special issue of Biological Rhythm Research. I feel privileged to dedicate this special issue to Prof. Waterhouse, who unfortunately left us for heavenly abode in October 2016, and could not witness the publication of this special issue. We shall always have the fond memories and warmth that Prof. Waterhouse always bestowed to many of us who have had the privilege of meeting him and discussing science. In particular, he a couple of times, when I used to work in UK in 1991, and when I visited UK in the year 2015, has hosted me, and I always felt home. Finally, it has been wonderful experience to be associated as Special (Guest) Editor for this special issue, and I take the opportunity to thank both Prof. Rietveld and all the contributing authors for immense cooperation in bringing out this volume.

Disclosure statement

No potential conflict of interest was reported by the author.