Unless you’ve been living under a rock, you’ve probably heard that adequate, high-quality sleep is essential for your physical, cognitive, and emotional wellbeing. But what you might be less clear about is why.

Turns out, sleep’s benefits can be attributed to a series of stages, during which your body and brain go through several distinct processes that are essential for sustaining your health. Far from being inactive, your body is actually working hard to support you in your sleep. Once you understand what these processes look like, you’re more likely to make sleep a real priority.

A Brief Overview of Sleep Stages

Once we’ve fallen asleep, our brains enter a sleep cycle that consists of two primary categories of sleep: non-REM (or NREM) sleep, which includes multiple distinct stages, and then Rapid Eye Movement (REM) sleep. Your body alternates between these cycles from the time you fall asleep to the time you wake up.

These stages may be influenced by a number of factors, including age, drug or alcohol use, the amount of sleep you obtained the night before, temperature and light, sleep or circadian rhythm disorders, and so on. In spite of these variations, most sleepers go through a predictable pattern. Here’s an overview of these different stages.

Non-REM Sleep

Stage One

This is the transitional stage during which your brain shifts from wakefulness to sleep. It starts after you close your eyes. Your brainwaves slow down, your heartbeat and breathing slow down, and your muscles relax. This light stage of sleep lasts for a few minutes.

Stage Two

After transitioning out of Stage One, sleep gets a little deeper in Stage Two. During this stage, your brainwave activity slows down (with occasional bursts of activity), your body temperature drops, and your heart rate slows down even more to prepare you for even deeper sleep. This stage typically lasts between 10 and 15 minutes.

Stage Three

This stage is typically referred to as deep sleep or slow-wave sleep (SLS). During SLS, you enter a deeper stage of sleep during which your body is focused on recovery: It gets to work on bolstering the immune system, repairing tissues, and building bone and muscle. Even though lots of work is taking place, your heartbeat, brain waves, and breathing all remain slow and your muscles stay relaxed. This deep stage of sleep typically lasts for 15 minutes or more. It often occurs for longer durations in the first half of a sleeping period. According to some sources, Stage Three sleep is followed by a similar Stage Four.

REM Sleep

Most people enter a REM sleep stage approximately 90 minutes after beginning a sleep cycle. Even though you’re still fully sleep during REM sleep, it’s often described as an active stage of sleep because a lot is going on in your brain and body. During this stage:

  • Your eyes move from side to side behind your closed eyelids (hence the name: Rapid Eye Movement sleep)
  • Your heart rate and blood pressure increase
  • Your breathing speeds up and may be irregular
  • You’re most likely to dream vividly during this stage (though dreaming can occur at any point during the sleep cycle)
  • Your arm and leg muscles are temporarily paralyzed as a means of protecting you from physically acting out your dreams
  • Your brain gets to work on storing memories, processing learned information, and balancing mood

This stage of sleep accounts for an estimated 20 to 25 percent of each sleep cycle and may get longer further into the night. The first REM stage may be as short as 10 minutes, while the last REM stage may last for close to an hour.

Your body needs to go through each of these stages several times in order to adequately recover, sustain healthy cognitive function, and so on. Once you understand how your sleep stages work, it’s easier to grasp why sufficient sleep is essential for overall wellbeing. So when you’re making your to-do list for each day, make sure to include plenty of time for NREM and REM sleep.