“The brain doesn’t know how to predict when they should be awake. “This is why people who wake up at different times every day may feel tired a lot,” Pelayo says. And when there’s a discrepancy between the homeostatic drive to sleep and the signal to sleep that comes from the circadian system, problems like jet lag and other disordered sleep occur. “They’re two complementary systems in the brain,” Pelayo says. They’re regulated by hormones, such as the stress hormone cortisol and the sleep hormone melatonin, which get secreted by the brain to send these wake and sleep signals to the body. These internal clocks are what drive us to feel sleepy at night and more awake in the morning (even, for instance, if you slept poorly the previous night, or pulled an all-nighter). Then there’s our circadian rhythm, our body’s biological clock, which syncs our body functions with environmental cues. For instance, if you stay awake for 24 or 36 hours instead of the typical amount of time you spend awake during a day, such as 16 or 17 hours, sleep-wake homeostasis is the mechanism that drives you to sleep longer and deeper once you do sleep. The homeostatic sleep drive affects how deeply we sleep, too. The longer we’re awake, the more our bodies crave sleep - and the longer we’re asleep, the more the body wants to wake up.

First, there’s the sleep-wake homeostatic drive. Two internal systems control when we sleep and when we’re awake.

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