What Are the Four Stages of Sleep?

You likely have a preferred bedtime routine, whether it involves making a comforting cup of tea, meditating, or catching a rerun of your favorite sitcom. Similarly, your body has a routine of its own for sleep.

Each night, assuming you’re getting around the standard 7 to 9 hours of sleep adults need, your body goes through multiple sleep cycles — usually four or five per night, each lasting between 90 and 120 minutes. Each sleep cycle is divided into four sleep stages, which equate to distinct levels and types of brain activity as you rest.

The stages run from light sleep to REM, but they’re not necessarily progressive — you can go back and forth between them, which is why you may find yourself going in and out of dreams throughout the night. Not every cycle looks the same, either. The order and frequency of the sleep stages change, as does how long they last; for instance, you spend more time in deep sleep during the first half of the night.

The four stages are separated into two broader categories: rapid eye movement (REM) and non-rapid eye movement (NREM), but they differ in ways much more complicated than how fast your eyes are moving. Here are the four stages of sleep, explained.

Stage 1: Lightest Sleep (NREM, N1)

Stage 1 of the sleep cycle is the lightest sleep you experience, and includes the transitional phase from wakefulness to sleep, as well as the first several minutes after you initially fall asleep. Throughout the night, this stage only accounts for approximately 5% of your sleep time, and though there are some minimal changes in brain activity as both the brain and body begin to slow, you’re not fully relaxed yet.

During stage 1 sleep, your brain transitions from producing alpha waves, which are associated with wakefulness, to slower theta waves. (You can think of brain waves as your brain’s pulse, which speeds up and slows down depending on your bodily needs.) As your brain waves shift in stage 1 sleep, your breathing and heart rate slow down. It’s easy to move around and wake up at this point — if you’re woken up during stage 1 sleep, it may not feel like you were even asleep yet.

Stage 2: Core Sleep (NREM, N2)

Stage 2 of sleep is still a lighter stage, but you’re no longer lingering between being awake and asleep. In fact, it’s considered the stage of “core” sleep, though that doesn’t mean it’s the deepest or most essential. During core sleep, the brain can create “sleep spindles,” which are brief bursts of electrical activity before its waves slow down. According to experts, those bursts could help with memory and information organization.

You’re not as likely to wake up during stage 2 as you are during stage 1, and your heart rate and breathing slow even more. Your body temperature may also drop. This stage typically lasts about 25 minutes (at which point you can move to either stage 3 or REM sleep) and accounts for around 45% of your overall sleep.

Stage 3: Deep Sleep (NREM, N3)

Stage 3 is when you finally enter deep sleep. This stage is also called slow-wave sleep, because the brain shifts to emitting mostly delta waves and physiological functions slow even more. This deeper level of sleep, which is much harder to wake from than stage 1 or 2, physically restores the body, reinforces the immune system, and helps clear the brain of accumulated neurotoxic proteins.

This stage is also critical for forming memories and learning new tasks. You spend more time in stage 3 sleep during the first half of the night than the second, and it accounts for around 25% of your total sleep in a night.

Stage 4: REM Sleep

REM sleep is the last stage of the cycle and makes up about 25% of your time asleep. This is the stage where most active dreaming occurs, and brain activity actually looks similar to how it does when you’re awake, emitting low-voltage, mixed-frequency beta waves. 

The REM stage is when memories and emotions are processed by the brain, and it may also be important for brain development and maturation. Usually, you spend more time in REM sleep during the second half of the night than the first.

It’s tough to wake up on your own from deep sleep, but you can wake up directly from REM sleep. When you do, you’re more likely to remember your dreams. You may also revert back to light or core sleep before you finally wake up in the morning. Then, later that evening when you return to bed, you start cycling through the sleep stages all over again.