The Four Stages of Sleep and What They Actually Do

January 27, 2024 Mike Zonta Leave a comment

Sleep better knowing how long each stage should last, what happens, and why you need to cycle through them several times

Published in

Dec 26, 2023 (medium.com)

This article is drawn from Chapter 9 of my book, Make Sleep Your Superpower. It has been revised and updated to better stand alone here.

You don’t need to become an expert on rapid-eye-movement (REM) sleep to figure out how to get some, but a cursory understanding of the primary stages of sleep will illuminate why it’s so important to cherish efficient slumber, and why good daytime habits are crucial to successful nighttime sleep.

Yet because sleep remains somewhat mysterious, experts describe the stages, and what exactly occurs during each phase, differently. I’ve researched the many different explanations and whittled things down to the essentials.

For practical purposes, a general summary goes like this:

After your head hits the pillow, it might take anywhere from five to 20 minutes to nod off. There’s no correct amount of time, but it’s considered unusual if you fall asleep instantly or take hours. Then, at some point, the transition is nearly instantaneous: You’re awake, you’re awake, you’re awake, blink, you’re out.

On a good night, you’ll go through the following four stages in this order, anywhere from about four to seven times, with each making up the very rough estimate of total sleep time noted.

Stage 1: Transition

This is a short period lasting just one minute to perhaps several minutes. Sleep is light. Breathing and heart rates slow but remain near normal. Muscles relax. Core body temperature, which has been falling during the evening, drops further. Brain waves slow, too. You can be easily awakened, perhaps peeved but likely without the grogginess you might feel if woken during other stages.

Estimated fraction of the total: 5%

Stage 2: Light sleep

Sleep begins to deepen. The heart and breath slow further. Body temperature falls and eye movement stops. Brief bursts of brain-wave activity occur. This stage lasts anywhere from 10 to 25 minutes, but after repeating all the stages multiple times. It’s thought that some memory consolidation begins, but that the brain remains somewhat vigilant for any threats. Think of Stage 2 as one you have to go through to get to the most important two stages. And on a bad night—if you drank too much or you’re packing a lot of anxiety—you might waffle in this stage longer than ideal.

Estimated fraction of the total: 45%

Stage 3: Deep sleep (also called slow-wave sleep)

The most restorative phase finally comes. Breathing is slow but regular, and the pulse has fallen off significantly. Muscles are relaxed. Body temperature has now fallen by 2 to 3 degrees Fahrenheit compared to its peak in late afternoon. Brain waves slow even further, hence this phase is referred to as slow-wave sleep.

It’s hard to wake you up during deep, slow-wave sleep, and you won’t like it if someone does — you may be foggy with sleep inertia—a sluggishness of mind and body—for up to an hour.

The first round of this relatively brief stage typically lasts longer than later rounds, so it’s crucial that when you fall asleep, your mind and body are ready to settle into this initial around of deep, restorative slumber.

The overall time spent in this slow-wave stage is crucial, since it’s when the brain’s glymphatic system—which removes misfolded proteins and other toxins and waste from the brain to literally clear your head—is most active, and the body gets to work repairing its tissues by releasing growth hormones and a whole suite of other chemicals critical to fixing, patching and sprucing up muscles, bones, organs, skin, the immune system and everything else you need to stay healthy and avoid aging faster than what’s natural.

Estimated fraction of the total: 25%

Stage 4: REM sleep

About 90 minutes after falling asleep, if all goes according to nature’s plan, the initial version of rapid-eye-movement (REM) kicks in. Breathing picks up and becomes irregular. Heart rate rises.

You’re doing most of your dreaming now—dreams that you might or might not remember. Your limbs become effectively paralyzed so that you don’t kick someone or punch yourself in the face. It’s thought that bones, muscles and other tissues continue to undergo repair during this phase. But most importantly, memories are consolidated—effectively stored on your brain’s hard drive—while extraneous information is cleared from the mind and emotions are processed and settled. This stage is crucial for managing stress and anxiety.

The initial round of REM might be 10 minutes, with later rounds lasting up to an hour, so the bulk of REM sleep happens in the second half of the night. That’s why sleeping long enough is so important, and why being woken by an alarm before you are ready can be so disorienting and leave you unrefreshed.

Estimated fraction of the total: 25%

It’s common to wake up after each round of the four sleep stages—often right after a dream—and perhaps roll over or otherwise shift position and go right back to sleep, and not even realize it.

If you need to pee, you should go—that’s normal. Going back to sleep is obviously good, because you want to keep those four stages cycling through their ideal four to seven sets. Likewise, waking up during the rounds — cutting a stage short, even if you don’t realize it — is bad.

Depth, quality and continuity of sleep through the stages — what I like to call overall efficiency — determines how restorative a night can be.

Written by Robert Roy Britt

·Editor for Wise & Well

Editor of Aha! and Wise & Well on Medium + the Writer’s Guide at writersguide.substack.com. Author of Make Sleep Your Superpower: amazon.com/dp/B0BJBYFQCB

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