The circadian rhythm, also called circadian clock or just inner clock, is the endogenous capability of an organism to retain a 24 h rhythm without being exposed to environmental stimuli that indicate the time of day. Animals, plants and even fungi generate a circadian rhythm and plays a major role in daily live.

For humans it is the mechanism to regulate sleep and wakefulness. Interestingly, during the first half year of life, as infants, humans do not have a working circadian rhythm, which explains why babies have an erratic sleeping pattern. They wake up in the middle of the night and are wide awake, much to the suffering of the parents. At around three to six months of age, the circadian clock starts to get established, and babies begin to sleep at night and are awake during the day.

How are sleep and wakefulness regulated?

There are two mechanisms or systems that take care of sleep and wakefulness. This first is the “sleep and wake homoeostasis”, which is a balance-system of sleep and wakefulness. It measures the ratio of both, and in case of insufficient sleep over a longer period of time the need for sleep will accumulate, resulting in the enhanced feeling of sleepiness. The sleep-wake homoeostasis system is also responsible for continuous sleep throughout the night.

The second regulator of sleep and wakefulness is the circadian clock, and it regulates the timing of both states. In general, the circadian clock’s strongest effect on sleep occurs between 2 and 4 o’clock at night, when people are in their deepest sleep. At this time, the body’s production of the sleep hormone melatonin is at its highest level.

Upon awakening in the morning, melatonin levels are strongly reduced, and instead stress hormone levels, such as cortisol, rise. This is also the time when the circadian clock ensures that the body’s digestive system is animated and that the brain starts to reach peak performance; ready for analysing, decision making, altogether ready for work.

In the afternoon, the circadian clock improves motor-control to ensure peak physically performance.

In the evening, the circadian clock prepares the organism for sleep by slowing down the metabolism, reducing stress-hormone levels and again increasing melatonin levels that reach peak levels between 2 and 4 o’clock at night; thereby closing the cycle.

Overall, the circadian clock gets the organism into a rhythm that attunes it for reoccurring events and regulates daily processes. But how is rhythm regulated?

Light and darkness are the key players here, especially the timing when they are perceived. Light, obviously, is sensed by specific photoreceptors of the eye’s retina. Studies in rodents found that entrainment of the circadian rhythm depends on melanopsin that has peak sensitivity to light of around 480 nm wavelength, and opsin of the cones that has peak sensitivity to light of above 500 nm wavelength (middle wavelength-opsin in the outer retina). Therefore, melanopsin perceive the blue light proportion of daylight and conveys blue light signals. The participation of other retina photopigments in entrainment is, however, not excluded.