In short, our brains cycle through four different sleep stages multiple times during the night, he says Michelle Drerup, PsyD, a psychologist and director of the Behavioral Sleep Medicine Program at the Cleveland Clinic. While your loved ones might describe you as a lump on a log when you’re passed out in bed, there’s a lot going on under your eye mask.
There are four distinct stages of sleep – three classified as non-REM sleep (NREM), followed by the fourth stage, REM sleep. dr Right at the start, Drerup adds a major caveat that researchers still don’t know a lot about what happens in our brains during sleep. Much of the work in this field has to do with theorizing what might happen when we rest based on studying sleep patterns and brain waves in patients in a sleep lab.
Here’s what researchers know so far about the four stages of sleep:
Stage 1 Non-REM (Rapid Eye Movement) sleep.
Stage 1 initiates the sleep cycle as we transition from being awake to being lightly asleep. This first phase is when you are about to drift off into slumber. Your heartbeat, eye movements, and breathing slow down; your muscles relax; and your brain activity begins to rejuvenate.
“We’re just starting to doze off at this stage. When someone wakes up, they may not even feel asleep,” says Drerup.
Stage 2 non-REM sleep
This phase is all about preparing for the coming deep sleep and REM sleep. Overall, your body temperature drops, your muscles completely relax, and your brain waves slow down into small bursts of electrical activity Eric Landsness, MD, PhDAssistant Professor of Neurology and Sleep Medicine at Washington University School of Medicine in St. Louis.
When the sensory nervous system (sight, hearing, taste, smell, and touch) shuts down for the day, sleep spindle activity indicates that memory processing of the day’s events is taking place in the brain.
“There is something very beautiful about that. They look like little spindles on a sewing machine — these are neurons that send messages from your short-term memory to your long-term memory,” says Landsness. This messaging process is thought to be what causes your brain to turn short-term memories into long-term memories, he explains.
According to Drerup, we spend most of our time in stage 2 sleep — about 50 percent of the night, about 20 to 60 minutes per cycle.
Stage 3 non-REM sleep
This final phase of non-REM sleep is called deep sleep, which our bodies rely on to feel refreshed in the morning. according to dr Cline, this is the phase when you are most disconnected from your waking life. Your heartbeat and breathing slow down the most during phase 3 because your body and muscles are fully relaxing and this is the most difficult time to wake up.
This crucial phase is all about restful sleep, physical recovery and strengthening the immune system. Deep sleep also refreshes the brain to store new memories the next day, Cline says.
Brain activity at this stage is characterized by so-called delta waves or slow wave sleep. Because this phase of deep sleep is the hardest time to wake you up, you may feel more groggy if you’re jolted awake than if you were awakened during the other sleep stages, Drerup says.
“Deep sleep is important for consolidating long-term memory—facts, events, geography, and spatial awareness,” he says Hussam Al-Sharif, MDPulmonologist and sleep medicine specialist at the Mayo Clinic in Eau Claire, Wisconsin.
We spend about 20 to 40 minutes in stage 3 deep sleep per sleep cycle.
Stage 4 REM sleep
The hallmark of REM sleep is in its name — rapid eye movements. During this fourth stage of sleep, your brain activity spins so much that it appears on brain scans as if you are awake. Heart rate, blood pressure and breathing also increase again. As your eyes dart back and forth, your muscles and body are paralyzed, Drerup says.
Memory consolidation also occurs during REM sleep. While in deep sleep the brain is thought to be processing new facts, places or formulas (e.g. from a textbook), in REM sleep the brain is thought to be processing abstract thinking and emotional content. As the brain replays the day’s events, it’s looking for emotional meaning, says Landsness.
Researchers theorize that dreaming occurs at all stages of sleep, but that our most vivid, narrative-like dreams occur during REM sleep because this emotional processing is taking place. And we tend to remember these dreams because we often wake up in this sleep phase in the morning.
“It appears that REM sleep is a way for our brains to deal with events that happened while we were awake, absorb new information we’ve learned, and process certain memories,” says Dr. Al Sharif.
At the beginning of the night, REM sleep can last just a few minutes, but from the second half of the evening until dawn it can stretch for up to an hour (more on this below). Overall, REM sleep accounts for about 25 percent of adult sleep.