The Anatomy of a Sleep Cycle

We know, it’s easy to assume that the amount of time you’re sleeping equates to getting enough sleep—especially when you’re having trouble drifting off. But while the number of hours does count, that’s only part of the picture. More importantly is the quality of sleep you’re getting—meaning how restorative to your mind and body it is. This is directly related to how smoothly you sail along stages of your sleep cycle as you traverse them during the night.

You may have heard of the “five stages of sleep.” But, in 2007, the American Academy of Sleep Medicine (AASM) consolidated the previously-accepted five stages of sleep into four based on scientific consensus on what the stages of sleep are.⁴ They all play an integral role in whether or not you wake up refreshed, as well as explain how certain sleep issues can affect your health and quality of life.

The average person goes through a total of four to six sleep cycles during the night, each one comprised of the aforementioned four stages: the first stage is known as “rapid eye movement” (REM), and the other three are non-REM (NREM), designated as NREM 1, NREM 2, and NREM 3. The higher the NREM sleep stage, the more difficult it is to be awakened. And just like every dream is different, the amount of time spent in each stage varies as sleep progresses, and can change from person to person, night to night. These variations depend on a number of factors from age to recent sleep patterns and food and drink consumed near bedtime, as well as specific underlying sleep issues. 

(Fun fact: During sleep studies, you can visually see these individual stages within the anatomy of a sleep cycle, a.k.a. “sleep architecture,” in a chart known as a “hypnogram,” which depicts brain wave activity on an electroencephalogram, or EEG.) 

Stage 1 of non-REM sleep, which typically lasts one to five minutes, is essentially when you start to drift off to sleep. Your body may not be completely relaxed yet, but activities within it and your brain start to slow down. If you’ve ever experienced brief, involuntary movements or “twitches” while you doze, this is the stage they likely occurred in. It’s also the stage where it’s easiest for your sleep to be disrupted. Without interruption, however, your drifting continues on to the following sleep stage.

In Stage 2, usually lasting 10 to 60 minutes during your first sleep cycle—and possibly longer with each subsequent one—the muscles in your body relax further, your breathing and heart rate slow, and your temperature drops slightly. And movement of your eyes usually stops altogether. Your brain activity slows as well, aside from short bursts of activity intended to help block out any distractions. All in all, people usually spend about half of their snoozing time in this sleep stage.

This is the widely coveted—and important—stage known as “deep sleep,” which shows clear “delta” brain waves, leading to its pseudonyms of “delta sleep” and “slow-wave sleep” (SWS). It lasts 20 to 40 minutes, and makes it even harder to be awakened. Your body is much more relaxed, as your muscles, breathing, and pulse all decrease to their lowest levels of your sleep cycle. 

Believed to be the most critical stage for restorative sleep, this stage may also support your immunity and other bodily systems, as well as your memory, creativity, and cognitive insights. As your sleep progresses throughout the night, each time you re-enter this stage, you spend slightly less time in it in exchange for more time in REM sleep.

Despite being fully asleep, for 10 minutes to an hour while in the REM sleep stage, your brain activity rivals levels of when you’re awake—which can explain why this stage is when you experience the most vivid dreams of your snooze. However, aside from jumpy, rapid eye movements, breathing (which can be irregular and rapid), and heart rate (which may be increased), the muscles in your body are totally relaxed—in fact, they’re in a state of temporary paralysis thought to protect you from acting out your dreams and inadvertently injuring yourself.

This sleep stage is considered vital for supporting cognitive functions, such as learning, memory, and creativity. As you progress through the stages of your sleep cycle, REM sleep gradually lasts longer and longer, making up about 25% of your total nighttime sleep.