We all know that our nightly sleep is made up of four (or five) distinct stages and we know a little about how those stages work. But how is it that we slip into sleep every night? And why are humans diurnal (rather than nocturnal like bats)? All this comes down to the circadian rhythm. Each of us has an internal clock that the brain uses to release certain chemicals at certain times and to control the body’s temperature.
Circadian rhythms allow organisms to anticipate and prepare for regular environmental changes. Organisms from the simplest prokaryotic cyanobacteria all the way to incredibly complex humans. This regulation helps internal metabolic processes to coordinate with their environment. In humans, the circadian rhythm runs approximately every 24 hours. This “clock” relies primarily on the secretion of different chemicals from the hypothalamus in the brain. The body absorbs stimuli such as light and dark cycles throughout the day and these stimuli signal the brain to release these chemical processes to induce sleepiness or wakefulness.
The key players in circadian regulation are melatonin which is secreted by the pineal gland and stimulates wakefulness or sleepiness (for example, the body begins secreting melatonin at approximately 9pm while melatonin in the body remains undetectable during the day); core body temperature dips and rises throughout the day depending on where int the cycle one is (for example, average human temperature is at its lowest at 5am, just two hours before regular wake time); finally, the plasma level of cortisol (another horomone secreted by the adrenal gland) is at its peak in the early morning as humans wake and at its lowest about 3 to 5 hours after sleep. Scientists have learned a great deal about the nature of circadian rhythms by studying these key markers.
When we pay attention to our natural circadian rhythm and don’t try to stress our system, we will have the best chance of having a good night’s rest. When circadian rhythms are purposefully or inadvertently suspended (this often happens in the case of international pilots who fly across time zones and are forced to sleep “off their clock”), this can result in fatigue, depression, exacerbation of physical diseases, and more.