Philip Lewis

University Hospital of Cologne, Germany

https://doi.org/10.36866/pn.113.5

2018 has become a year for light, time and chronobiological discourse in science and society. Thus, we considered it timely to publish this themed issue to educate and inspire chronobiological thinking amongst physiologists at all career stages.

In August, citizen surveys and a European vote to abolish the biannual clock time change indicated a preference for the advanced summer clock all year round. Consequences include a lack of early morning daylight experienced in winter time, especially on the edge of time-zones and in more northern latitudes where the sun would not rise until well after 9 am. As morning daylight is a key time cue for the body’s circadian system, which temporally organises physiology, children and adults getting up and going to school or work in darkness may suffer continuous circadian misalignment throughout the winter period. In the short-term, we may expect increased traffic and workplace accidents, and poorer performance in schools due to debilitating effects on cognitive and physical performance, and sleep impairment. In the long-term, significant debilitating effects on metabolic and psychiatric health would be expected given the important co-governing roles of the circadian system on metabolic and psychiatric physiology.

Also in August, the International Agency for Research on Cancer (IARC) called for data relevant to the proposed carcinogenicity of shift-work that causes circadian disruption. The current Group 2a (“probable”) classification is to be reviewed in Lyon in June 2019. Importantly, significant portions of the population (around 21% of the EU workforce) are engaged in shift work which can be disruptive to chronobiology and sleep. “Living for the weekend,” whereby people tend to stay up later and sleep in later than on work days, can be equally disruptive to circadian systems. Moreover, there is increasing 24–hour access to various amenities which, along with light at night from light-emitting devices such as smartphones and tablets, can also throw biological rhythms out of sync with each other and contribute to what is already considered a sleep deficiency pandemic.

In October, the Chinese government announced a proposal to launch satellites in 2020 that will provide a continuous full-moon-like light glow over the Chengdu region by reflecting sunlight similar to the moon. Just as daylight is important for entraining a strong and stable circadian rhythm, so is darkness. A change over from light to dark also provides the body with a sense of time, such as indicating the appropriate sleep period for humans or wake period for nocturnal animals. The impact of continuous full-moon-like light at night on circadian and lunar-light rhythms of humans, other animals, and plants is unprecedented.

From a physiological perspective, we have chronobiological co-governance of large portions of the genome and biochemical processes in every cell as well as chronobiological communication within and between tissues and organs. All of this provides a temporal organisation to physiology in response to ubiquitous environmental time cues such as the daily light-dark cycle and meal times. Such extensive co-governance of physiology means the contribution of disrupting circadian physiology to the global burden of disease could be grossly under-appreciated. Metabolic and mood disorders, and cancer may just be the tip of the iceberg.
But there may be light at the end of the tunnel.

Such an extensive system of co-governance also means that appropriate timing of medical interventions could significantly impact efficacies or decrease side-effects. Understanding how perturbation to, or reinforcement of, circadian rhythm could impact physiology could have significant health-care and economic impact. Importantly, chronobiology is ubiquitously applicable to physiology sub-disciplines. Thus, physiology can have a massive role to play in the chronobiology field going forward. Furthermore, chronobiological and sleep concepts are readily understood by a non-scientific audience. For instance, the association of sleep–wake cycles with day and night is conspicuously recognisable: folk wisdom will tell us to avoid eating large meals late at night, and exacerbations of diseases are often associated with time of day, even to patients of the common cold. As such, advances in chronobiology may be quickly and easily translated to benefit individuals and populations.

The content herein – including chronobiology history, education, prevalent issues, contemporary studies, philosophical questions, collaboration, and translations of research – will not disappoint. Given the hot topic nature of this field, near universal applicability for physiological research with a clock in every cell, and that every individual in the world is exposed to light, dark, and other circadian time cues, it is both a pleasure and a privilege to be Guest Editor for this Chronobiology and Sleep Special Issue of Physiology News, and to fittingly end the year in line with The Society’s Theme for 2018.

Heightening focus on chronobiology now could be the stitch in time that saves nine!