The circadian rhythm governs a 24-hour cycle in wakefulness, sleepiness, body temperature, metabolism, immune function, and mood, and is synchronized with the external light-dark cycle.

“The circadian rhythm is a 24-hour rhythm in all sorts of functions. The most prominent one is a rhythm in our feelings of wakefulness and sleepiness. You also have a rhythm in sleepiness and wakefulness that correlates with that. We tend to be sleepy as our temperature is falling, getting lower. And we tend to be more awake or waking when our temperature is increasing.”