The circadian rhythm is our biological clock. It has 24 hour cycle telling us when to sleep, when to rise and when to eat. This biological clock is not just found in us humans — all living things, right down to microbes, rely on a circadian rhythm for their daily functioning.
For the most part, we sleep when it’s dark and do stuff during daylight hours, because most of what we do is better done in the light of day. But there’s also a physiological reason why we get tired at night and want to sleep. Our clocks are linked to the light-dark cycle. When our eyes are exposed to daylight in the morning, our brain sends out messages to raise our body temperature and heart rate, produce hormones like cortisol and stop releasing others like melatonin, and so we wake up.
As daylight diminishes, our eyes send reverse messages to the brain — lower the body temperature and heart rate and begin to secrete melatonin, and we become sleepy. The light-dark cycle is important — we experience jet lag, especially moving west to east, partially because you can’t sleep well on a plane, but also because we’re getting exposed to daylight before we’re supposed to.
The strongest sleep and wake triggers are about 11 p.m. and 7 a.m., respectively. But there can be some variation, depending on whether you’re a “morning” or “night” person. And our circadian rhythms dip and rise, so we feel periods of alertness and sleepiness throughout the day. We’re most sleepy between 2-4 a.m. and again between 2-3 p.m. and most awake in the early morning and late afternoon. How sleepy we feel in the afternoon can depend on how well we’ve slept the night before. Here’s some specifics on body functions throughout the 24 hour cycle: