All life on Earth evolved under light and dark cycles. In sunlight, we can see clearly, while at night, there is darkness with limited visible light. But more than vision, light cycles are important to regulate our whole body physiology, generating daily rhythms. The temporal programme found in organisms from all phyla is called “circadian clock”, which is derived from the Latin words circa and dies, meaning “approximately a day”. The circadian clock is an adaptation to Earth’s rotation, conferring a 24h structure on process at all levels – from genes to behavior – even in the absence of daily signals. It is because of it that we feel sleepy at night and stay awake during the day, to cite the most obvious example.
Each organism functions like a network of multiple interactions, in uncountable ways. Scientists’ work is to decipher the components and steps of these interactions. Light information is received by a specific area of the brain (the suprachiasmatic nucleus), via the retina. With this information, this brain’s nucleus synchronizes its own “clocks” and sends the information for a network of other organs’ clocks. The main substance that marks rhythms in every known animal is a neurohormone called melatonin. Melatonin is produced every night, in response to darkness, by the pineal – a tiny organ in our brain – and has many roles in regulating almost all body systems and organs.
As humans evolved being exposed to light/dark cycles, it is not surprising that human physiology is profoundly affected by the daily and seasonal changes in light exposure. Exposure to the appropriate time of light during the day and evening enhances human health, well being, immune response and productivity. However, exposure to light sources that do not match natural solar spectrum to the time of day or evening – as we’re doing in our modern way of living – is harmful to human health. We have been exposed to artificial light many more hours than natural light, and we also expose ourselves to artificial lights of TVs, computer and cell phones in the middle of the night. Besides, it’s not rare to find workers that switch night and day, or that travel long distances in a few hours, rapidly switching from one time zone to another, changing the timing of their internal clock.
These situations to which we are being exposed can alter hormones secretion and body perception of day/night cycles. Epidemiological studies have been discovering some relationships between circadian clock alteration and human disease. For instance, there’s an increased incidence of cancers, as well as metabolic and gastrointestinal diseases, in long-term shift workers. Melatonin might have a central role on these diseases development. Within the effects of regulating organism’s systems, melatonin has anti-tumoral effects. Since it’s produced at night and inhibited at light’s presence, people exposed to light-light cycles don’t produce melatonin and are possibly more susceptible to oncogenesis.
Circadian clocks can also regulate and be regulated by other physiological and behavioral aspects, like exercise or feed’s time and regularity. One behavioral aspect that can be harmful for circadian clock regulation and human health is the regular use of alarm clock. This habit usually leads to sleep deprivation and induces the individual to perform day activities when one should be sleeping. Almost everybody knows some of the negative effects that can be observed when we sleep less than the optimal timing: we can feel lethargic and exhausted, and sometimes also feel pain and cognitive deficits. Curiously, increases in heart attacks are observed after daylight savings time changes. Despite these epidemiological data, we should remember that the circadian system, predisposition and vulnerability to pathologies as well as their combination are all highly individual.
The circadian clock is part of physiology and also modulates it throughout the day. Several detailed studies are being performed to better understand how light/dark cycles regulate animal’s physiology, through metabolism, cellular and genomic level. Some of these studies, as well as some alternatives to reduce circadian clocks damages caused by our modern style of living, will be topics of future articles in this blog!
By Bruna de Oliveira Cassettari.
Roenneberg, T. & Merrow, M. The Circadian Clock and Human Health. Curr. Biol. 26, R432–R443 (2016).