All About Sleep Cycles and the Stages of Sleep

From physical restoration to memory consolidation to dreaming, there’s a lot going on in the body and brain when we power down for the day and go to sleep. As you sleep, your brain cycles through four separate sleep stages, each with distinct patterns of activity and each playing a pivotal part in helping you get a good night of slumber.

That is why sleep quality, in addition to sleep quantity, is important. It’s our ability to progress through the various sleep stages (and spend enough time in the deepest ones) that determines whether we’re getting high-quality rest.

“Think of sleep in a way as similar to nutrition, where we want to get a balanced set of sleep stages (just like you want to include a balance of different micro- and macronutrients in a healthy diet), as each seems to serve an important function,” says John Cline, PhD, an assistant professor of clinical psychology at the Yale School of Medicine and fellow of the American Academy of Sleep Medicine. Cycling through all the sleep stages enough times makes for optimal physical, emotional, and cognitive health, he explains.

Here’s a closer look at the stages of sleep that make up a sleep cycle and why each stage is so critical for good health and ensuring that we wake up feeling refreshed and well rested.

In a nutshell, our brains transition through four different stages of sleep several times throughout the night, says Michelle Drerup, PsyD, a psychologist and director of the Behavioral Sleep Medicine Program at the Cleveland Clinic. So while your loved ones may describe you as a lump on a log when you’re passed out in bed, there’s a lot going on underneath your eye mask.

There are four unique sleep stages — three that are classified as non-REM (NREM) sleep, followed by the fourth stage, REM sleep. Dr. Drerup adds a big caveat right off the bat that there’s still a lot that researchers don’t know about what happens in our brains during sleep. A lot of the work in the field has to do with theorizing what may be happening when we’re resting, 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 kicks off the sleep cycle, as we transition from wakefulness to a light sleep. This first stage is when you’re just drifting off to slumber. Your heartbeat, eye movements, and breathing slow down; your muscles relax; and your brain activity begins to taper off.

“We’re just starting to doze off in this stage. If someone wakes up, they may not even feel like they were asleep,” Drerup says.

Stage 2 Non-REM Sleep

This stage is all about preparing for the deep sleep and REM sleep to come. Overall, your body temperature drops, your muscles fully relax, and your brain waves slow to little bursts of electrical activity, according to Eric Landsness, MD, PhD, an assistant professor of neurology and sleep medicine at the Washington University School of Medicine in St. Louis.

“There’s something very beautiful about it. They look like little spindles on a sewing machine — these are neurons sending messages from your short-term memory to your long-term memory,” Landsness says. That messaging process is thought to be how your brain turns short-term memories into long-term ones, he explains.

Drerup says we spend the most time in stage 2 sleep — about 50 percent of the night, for about 20 to 60 minutes per cycle.

Stage 3 Non-REM Sleep

This final stage of non-REM sleep is categorized as the deep sleep our bodies rely on to feel refreshed in the morning. In this stage, you’re most disconnected from your waking life, according to Dr. Cline. Your heartbeat and breathing slow down the most in stage 3, as your body and muscles fully relax, and it’s hardest to be awakened during this time.

It’s all about restorative sleep, physical recovery, and bolstering the immune system during this crucial stage. Deep sleep also refreshes the brain for encoding new memories the next day, Cline says.

Brain activity in this stage is by marked by what’s called delta waves, or slow-wave sleep. Because it’s hardest to wake you in this stage of deep sleep, if you are stirred awake, you might feel groggier than you would if awakened during the other sleep stages, Drerup says.

“Slow-wave sleep is important for consolidating long-term memories — facts, events, geography, and spatial sense,” says Hussam Al-Sharif, MD, a pulmonologist 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 movement. In this fourth sleep stage, your brain activity revs up so much that it looks like it’s awake on brain scans. Your heart rate, blood pressure and breathing pick up again too. While your eyes dart back and forth, your muscles and body are paralyzed, Drerup says.

Memory consolidation also happens during REM sleep. While during deep sleep the brain is thought to be working through new facts, locations, or formulas (say, 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 will look for emotional meanings, Landsness says.

Researchers suspect that dreaming occurs in all stages of sleep, but that our most vivid, storylike dreams occur during REM sleep because this emotional processing is going on. And we tend to remember these dreams because we often wake up in the morning during this stage of sleep.

“It seems REM sleep is a way for our brains to deal with events that happened in wake time, absorb new information we learned, and process certain memories,” Dr. Al-Sharif says.

At the start of the night, REM sleep may last for just a few minutes, but from the second half of the evening until dawn it can extend for up to an hour (more on that below). Overall, REM sleep accounts for about 25 percent of sleep in adults.

A sleep cycle is when your brain transitions through each of these sleep stages throughout the night. If you’re getting a solid seven to eight hours of slumber, you can count on going through about four to six sleep cycles, according to Cline.

In the first go-round, you’ll pass through the stages of sleep in chronological order. But after that, your sleep cycle won’t necessarily restart at stage 1 again. You may transition from REM sleep directly into stages 2 or 3. REM sleep typically leads back into stage 2 though, Cline says.

That said, as you move through your sleep cycles, you might restart at stage 1 again and even wake up, but the awakenings are typically brief enough that you don’t even remember them, Landsness says.

Each sleep cycle typically takes lasts about 90 minutes, but it can extend to as long as 120 minutes, according to Al-Sharif.

The composition of a sleep cycle changes throughout the night too. During the first two cycles, people tend to spend more time in stage 3 deep sleep, but in the latter half of the night and into the morning hours, we may be mainly in REM sleep, Cline says.