What Happens After You Close Your Eyes? Sleep Cycles Explained

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What Happens After You Close Your Eyes? Sleep Cycles Explained

In the hours after you crawl under the covers and before the alarm goes off, your body shifts among five distinct sleep cycles. During each sleep cycle, your brain goes through multiple patterns of activity that are important for your health.

“Sleep isn’t a uniform state,” says Jeffrey Iliff, PhD, assistant professor at Oregon Health & Science University. “Your brain is not simply asleep or awake; it is passing in and out of different sleep states.”

Here’s a guide to the five different sleep cycles to help you understand what really happens after you fall asleep.

STAGE 1

This is the shallowest stage of sleep, which Iliff refers to as “the phase between waking and sleeping.” Your brain produces alpha and theta waves that are associated with relaxation. It’s common to experience sudden muscle jerks or a feeling of falling during stage 1 sleep.

STAGE 2

Theta wave activity continues. The most noticeable difference between stage 1 and stage 2 sleep are the phenomena of sleep spindles (increases in theta wave frequency) and K complexes (increases in theta wave amplitude). Your heart rate slows and body temperature drops during this stage.

STAGES 3 AND 4

These two stages are lumped together under the umbrella of “slow wave sleep” and are characterized by large parts of the brain turning on and off. This on/off rhythm happens about once per second and is a phenomenon known as up states and down states.

Stages 3 and 4 mark the beginning of falling into deep sleep. There are no muscle or eye movements during these slow wave stages; your body temperature drops, blood pressure falls and your breathing slows. During these stages, Iliff notes that the body repairs muscles and tissues, boosts its immune function and builds energy stores.

Stages 1 through 4 are called non-rapid eye movement  sleep.

REM SLEEP

The most talked-about sleep stage is rapid eye movement.

It takes about 90 minutes to enter REM sleep, and each stage lasts about an hour; REM is the longest of the sleep stages. During REM sleep, your brain is more active.

“On brain scans, REM sleep looks a lot like being awake,” Iliff says.

Your heart rate and blood pressure increase, breathing is faster and more shallow and the namesake rapid eye movements occur. REM is also the sleep state when dreaming occurs. During this stage, your brain is also consolidating and storing long-term memories.

Iliff likens sleep cycles to the interval setting on the treadmill: One entire sleep cycle lasts about 90 minutes, which means the average person will go through all five stages 3–4 times per night.

When something interferes with your sleep — like insomnia or chronically staying up too late — it affects these cycles. You’ll still go through all five sleep stages, but a shorter sleep duration means you can’t cycle through them enough times to reap the benefits.

“Your brain is doing a lot of important stuff when you sleep, and you have to make sure you get enough of the right kind of sleep,” Iliff says. “You need to give your body’s natural processes enough time to take over so you don’t have a negative impact on your long-term health.”

During your nightly date with the sandman, be sure to give your body enough time to move through the sleep stages for the sake of your health.


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