How Does the Body Regulate Sleep?

“Homeostasis” describes a state of equilibrium between different elements of an organism or group. Sleep/wake homeostasis balances our need for sleep, called a “sleep drive” or “sleep pressure,” with our need for wakefulness. When we’ve been awake for a long period of time, our sleep drive tells us that it’s time to sleep. As we sleep, we regain homeostasis and our sleep drive diminishes. Finally, our need for alertness grows, telling us that it’s time to wake up.

If sleep/wake homeostasis alone regulated our sleep drive, we’d likely find ourselves yo-yoing between sleep and alertness throughout each day. We’d also likely feel most alert in the morning, with that alertness wearing off the longer we were awake. Instead, we can feel just as alert at 4:00 p.m. as we might have felt at 10:00 a.m., even when we’ve been awake for hours. That’s because sleep/wake homeostasis doesn’t work alone in regulating our sleep schedule; our circadian rhythm also plays a role.

How does our body clock know what time of day it is? The circadian biological clock is controlled by a part of the brain called the Suprachiasmatic Nucleus (SCN), a group of cells in the hypothalamus that respond to light and dark signals. When our eyes perceive light, our retinas send a signal to our SCN. The SCN sets off a chain reaction of hormone production and suppression that affects body temperature, appetite, sleep drive, and more.

Each morning, as sunlight creeps in, our body temperature begins to rise and cortisol is released, increasing our alertness and causing us to wake up. In the evening, as it becomes dark outside, melatonin levels rise and body temperature lowers. Melatonin stays elevated throughout the night, promoting sleep. As long as our eyes perceive light, the SCN responds by suppressing melatonin production. This explains why evening exposure to light, such as that from indoor light or electronic devices that emit blue light, such as a computer or television, make it harder to fall asleep.