Have you ever woken up just minutes before your alarm goes off and marveled at your body’s sense of time? Humans (and most living creatures) have an internal clock that mirrors nature’s cycles of day and night.
Nestled deep in an area of the brain called the hypothalamus, this timekeeper regulates many of our body’s functions, such as sleep, energy, and hunger.
Sunlight detected by cells in the retina of the eye sends messages to the brain that keep us in a roughly 24-hour pattern. These light cues trigger all kinds of chemical events in the body, causing changes in our physiology and behavior. For example, as evening approaches and the light in our environment dwindles, the hormone melatonin begins to rise and body temperature falls—both of which help us to become less alert and more likely to welcome sleep. With the help of morning light, melatonin levels are low, body temperature begins to rise, and other chemical shifts, such as an uptick in the activating hormone cortisol, occur to help us feel alert and ready for the day.
Stephan FK, Zucker I. Circadian rhythms in drinking behavior and locomotor activity of rats are eliminated by hypothalamic lesions. Proceedings of the National Academy of Sciences. 1972; 68(6):1583-1586.