NREM Sleep: Stages & Measurement

Instructor: Sharon Linde

Sharon has a Masters of Science in Mathematics

Sleep is something we all do, hopefully every day. Are you familiar with the cycles of sleep? Brush up on your REM's and NREM's in this lesson. Don't worry, no napping required.

Sleep Basics

Like food and water, sleep is a vital part of staying alive. After a good night's sleep, we're more productive, alert, and ready for the day. When it comes to sleep, both quantity and quality count. The average adult should get around eight hours of sleep a night. What happens if you don't get enough sleep? Some of us simply get cranky, but more dramatic things can happen as well. Sleep is the time for our bodies to heal muscles, release hormones, and regulate growth, among other things. Lack of sleep impairs our ability to think clearly and move our bodies efficiently, and can even delay or be detrimental to growth and development.

So what is a good night's sleep? Let's take a look.

Sleep Cycles Explained

Like we said, adults should be getting an average of eight hours a night. But it isn't just quantity that matters. The kind of sleep you're getting is just as important. During the night, we cycle through different stages of sleep. These sleep cycles follow a pattern of rapid eye movement, or REM, and non-rapid eye movement, or NREM. The cycle repeats about every 90 minutes.

REM Sleep

Approximately 25% of your night is taken up by REM sleep. Your first REM cycle begins around 90 minutes after falling asleep and then repeats about every 90 minutes throughout the night, getting a little longer at each stage. REM sleep is the time you dream and your eyes are shifting back and forth. REM sleep also boosts energy to your body and brain by increasing cortisol, a hormone that allows you to be alert in the morning.

NREM Sleep

The rest of the night, about 75%, is spent in NREM sleep. NREM is characterized by three cycles, or stages.

Stage One

You first enter NREM when you fall asleep. This stage is sometimes called somnolence, or drowsy sleep. This stage includes light sleep with active muscles - twitches or sudden jerking movements. Sometimes the eyes may flutter or roll. Have you ever had the sensation of waking suddenly as you're just falling asleep, almost as if you caught yourself from falling? This is called hypnic jerks and is common during this stage. This stage is short, around 10 minutes or 5% of sleep time, and is an easing in to deeper, longer sleep.

Stage Two

This stage is the true beginning of sleep. In this stage you are completely unaware of surroundings. The heart rate and breathing are regular and the body temperature drops. Because you'll pass through the stages frequently throughout the night, you spend the most time in stage two; it accounts for around 45% to 50% of the cycle.

Stage Three

This is truly the deepest stage of sleep. Physically, your blood pressure decreases and breathing slows down significantly. Muscles are at the most relaxed state as the blood supply to them increases and restoration of muscles and tissues occurs. During this stage, important hormones responsible for growth are released. A person in this stage is non-responsive to the outside world. This stage is longer than the others and happens in longer stretches during the first half of the night.

A full sleep cycle, from stage one through REM, lasts an average of 90-120 minutes. After stage three, we progress to REM sleep, typically marked by dreaming and rapid eye movement. REM lasts for about 25% of the total night, or 90-120 minutes. NREM sleep accounts for 4-7 hours a night, about 75% of the total night.

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