You may not remember everything that happens each night when you’re asleep, but if you’re doing it right, there’s a lot going on in your brain and your body, Pelayo says. “There are differences between sleep and awake for every single body system, but nothing as dramatic as the changes of consciousness during sleep, the brain function,” he explains.
The Different Stages of Sleep
During sleep the brain cycles, repeatedly, through different stages of sleep: (12)
Stage 1 Non-REM (Rapid Eye Movement) Sleep The first stage is when you’re actually falling asleep — stage 1 non-REM. Your heartbeat, breathing, and eye movement start to slow down and your muscles relax. Your brain waves are also slowing down and it’s still very easy to be awoken during this preliminary stage of sleep. (3)
Stage 2 Non-REM Sleep The second stage is when heart rates drops and body temperature falls even more. Eye movement stops completely and brain activity slows way down, other than brief bursts of activity.
Stage 3 Non-REM Sleep Next comes deep sleep. This is the stage of sleep that is heavy and restorative. Your heartbeat and breathing slow down the greatest during this stage of sleep and it is most difficult to be woken up.
REM Sleep Finally comes REM sleep, when your eyes begin to dart quickly back and forth from side to side (even though your eyelids are still closed). Brain activity speeds way up, closer to the amount of activity that happens when you’re awake. And this is the stage of sleep when most of your dreaming happens. Your breathing gets quicker and irregular during REM sleep. Heart rate and blood pressure start to go back up nearer to the speed they function at when you’re awake, though the muscles of your arms and legs become temporarily paralyzed during REM sleep. Sleep experts suspect this paralysis is a mechanism our bodies developed to help protect us from injury or other harm that might otherwise ensue if we were able to “act out” our dreams. (14)
Each cycle of sleep (which consists of all of the stages) usually takes about 90 minutes. And most people tend to spend more time during each cycle in deeper sleep earlier in the night — and more time in REM sleep later on. Each stage of sleep is important and both deep sleep and REM sleep play critical functions in terms of the learning and memory consolidation processes that happen during sleep. (15)
What Drives Sleep
Two internal systems control when we sleep and when we’re awake. First there’s the sleep-wake homeostatic drive. 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. The homeostatic sleep drive affects how deeply we sleep, too. For instance, if you were to 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.
Then there’s our circadian rhythm, our body’s biological clock, which is what helps sync our body functions with environmental cues. 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 even pulled an all-nighter). 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.
“They’re two complementary systems in the brain,” 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 occurs.
“This is why people who wake up at different times every day may feel tired a lot,” Pelayo says. “The brain doesn’t know how to predict when they should be awake. It’s like being constantly jet-lagged.”
The more sleep researchers learn about these two systems that control sleep, the more it is clear why not only getting enough hours of sleep, but also having good sleep habits (such as going to sleep and waking up at the same time each day) is important.