Most of us know the feeling. On a hot, sticky night, you kick off the covers, flip the pillow to the cool side, and still can't drift off. Then the temperature drops, and suddenly you sleep soundly. Is it just comfort, or is something deeper going on?
The short answer: your body temperature naturally rises and falls on a daily schedule, and it dips at night as part of getting ready for sleep. Sleep experts recommend a bedroom that is quiet, very dark, and cool, because noise, light, and uncomfortable temperatures have been shown to disrupt continuous sleep. A cool room works with your body's nightly rhythm instead of against it.
Your Built-In Thermostat
Deep inside your brain is a tiny master clock called the suprachiasmatic nucleus. It keeps your body on a roughly 24-hour cycle known as the circadian rhythm. That clock controls the timing of many things, including when you feel sleepy, when you get hungry, and your body temperature.
Over the course of a day, your core temperature gently swings up and down, roughly between 97.2 and 99 degrees Fahrenheit. It isn't fixed at 98.6 all the time. In fact, people kept in a bunker with constant light or darkness still show regular daily rises and falls in body temperature.
Cooling Down for the Night
As evening turns to night, your body begins to wind down. During the deep stage of sleep, called slow-wave sleep, your body temperature and heart rate fall, and your brain uses less energy. For a healthy young adult on a natural schedule, body temperature typically hits its lowest point around 6 a.m.
Researchers say good sleep timing happens when two signals line up in the second half of the night: the peak of the sleep hormone melatonin and the lowest core body temperature. In other words, a cooler body is part of the recipe for solid sleep.
The Dream Stage Twist
There's another wrinkle. Sleep comes in two main types: non-REM sleep and REM sleep, the stage when most dreaming happens. REM sleep tends to happen more when your body temperature is at its daily low.
During REM, many of the body's usual balancing systems are suspended. Your muscles go slack, and the body doesn't regulate itself in the usual way. Scientists are still sorting out exactly how all of this fits together. But studies are clear on one thing: uncomfortable temperatures can break up your sleep and leave you waking more often than you'd like.
Making Your Bedroom Sleep-Friendly
The classic advice for a better night is simple: keep the room quiet, dark, and cool. Other common tips include choosing comfortable bedding and hiding the bedroom clock, so you don't lie there watching the minutes tick by. Those last suggestions have less research behind them, though.
Some habits have stronger evidence. Avoiding noisy surroundings, skipping alcohol in the hours before bed, steering clear of mentally demanding tasks right before sleep, and not trying too hard to fall asleep all seem to help.
So on the next warm night, crack a window, run a fan, or nudge the thermostat down a bit. Your body is already trying to cool off for sleep, so give it a hand. And if poor sleep becomes a regular problem, talk to your doctor. Sweet, cool dreams.
