A circadian rhythm is a 24-hour cycle dictated by the body's internal clock, which is controlled by several physiological mechanisms. Most plants and animals have a circadian rhythm, and in humans it affects when people feel sleepy, wake up and want to eat.

The National Institutes of Health found that the body's master clock is controlled in an area of the brain called the suprachiasmatic nucleus (SCN), located inside the hypothalamus (the human brain's control center). The SCN receives different signals from the body, which it then responds to to set the body’s clock.

"The SCN is affected by various external cues, such as temperature and exercise, but it is most sensitive to light and therefore any changes in light impact on the signals sent by the SCN to the rest of the body which regulate the circadian rhythm," said Maja Schaedel, a clinical psychologist and co-founder of The Good Sleep Clinic in the U.K. "This then plays a crucial role in our sleep-wake cycle which is how our body ensures we are able to sleep at night and be alert and awake during the day," Schaedel told Live Science.