Sleep Cycles

Once, sleep was considered an inactive state in which body and mind “turned off” to rest.

Actually, quite the opposite is true. During certain stages of sleep, the brain is even more active than during the day.

There are 4 stages of sleep: 3 stages of non-REM (NREM) and 1 REM (rapid eye movement) stage.

Normal sleep cycle is: NREM 1 - NREM 2 - NREM 3 - REM.

This cycle usually lasts 90-120 minutes and continues with NREM 1 of the next cycle.

Typically, an individual goes through 4-5 sleep cycles per night.

Most NREM sleep occurs early in the night and the length of REM periods increases as the night goes on. That’s why there’s a good chance you’ll awaken from a dream in the morning.

Brain waves frequency and the purpose of each stage is very different, so let’s analyze them.

This is the first stage of sleep. It typically lasts 1-7 minutes.

It is the lightest stage of sleep. Muscles are relaxing, brain wave frequency is slowing down. There is slow rolling eye movement.

Occasionally, there can be hypnagogic jerks or a sense of falling down, while drifting in and out of NREM 1.

This is a much deeper sleep state than NREM 1. It usually lasts 10-25 minutes.

Awakenings are much less frequent, eye movement slows down, as well as the brain wave frequency. Body temperature and heart rate drop as well.

Throughout this stage, there are bursts of rapid brain activity, known as sleep spindles.

Purpose of NREM 2 is to protect the brain from waking up during sleep and preparing the body for deep sleep.

NREM 3 is the deep sleep stage. Typical duration is 20-40 minutes.

In this stage, brain waves have higher voltage but slower frequency - also known as delta waves.

Brain becomes less responsive to outside stimuli and it becomes more difficult to wake up the individual.

This is when the body repairs muscles and tissues, stimulates growth, boosts immune function, and builds up the energy.

This is why deep sleep is crucial for our health. Not having enough of it can lead to many problems.

If you want to take naps during the day, it’s best to wake up before the NREM 3, as this stage reduces the sleep drive. So - daytime naps should last 30-60 minutes.

Before the REM phase there is usually one more NREM 2 phase, which can last up to 15 minutes.

First REM stage usually happens 90 minutes after falling asleep and lasts around 10 minutes. Later REM stages last longer. Last one can take up to an hour.

Average adult has 4-5 REM stages during the night.

Brain becomes more active and this is where dreams occur.

Heart rate and blood pressure increases. Breathing becomes fast, irregular and shallow.

Eyes jerk rapidly in different directions, hence the name.

During REM phase, muscles are temporarily paralyzed. This is to prevent us from acting out our dreams.

Brain wave frequency increases to the levels experienced when we are awake - the alpha waves.

Dreams occur during each REM phase, but we cannot remember every dream upon waking up. If a person wakes up during the REM phase, he will remember the dream more often.

To sum up, here are the brain waves that define each stage.

Awake

Beta

NREM 1

Alpha

NREM 2

Theta

NREM 3

Delta

REM

Alpha