Image: Pexels/Ron Lach

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…