Basics You Should Know for Sleep Stages and Sleep Cycles
How Sleep Works

Sleep Stages and Sleep Cycles: Basics You Should Know

Here we’ll uncover all the information about sleep stages. Let’s give you a simple summary fist. You always pass through five stages during sleep, Stage 1,2,3,4 and REM sleep. Stage 1-4 are also called Non-REM sleep. These sleep stages repeat and cycle from 1 to REM. A complete sleep cycle has a pattern like this: Wake, Stage1, 2, 3, 4, 3, 2, REM, and it takes an average of 90 to 120 minutes. Each single stage last 5 to 15 minutes. The first sleep cycle has relatively a short period of REM sleep and a long period of deep sleep. And in the following cycles, the REM sleep extends and the deep sleep time decrease correspondingly.

Let’s dive right in and show your all the details.

What Are Five Stages Of Sleep?

Normally, the sleep process can be divided into five stages, Stage1, 2, 3, 4, 5. The first four stages are named Non-REM (NREM) sleep and the last stage is named REM sleep. When you get sleep, you cycle between Non-REM sleep and REM sleep. Sometimes short periods of wakefulness occur intermittently in the series of sleep stages.

Waking

Waking is a process in which the body prepares for sleep, so it is also referred to as relaxed wakefulness. As we know, when we go to bed, our muscles are still tense and brain wave activity is keeping highest. Then after a while, normally, we become sleepier, our body is slowing down, muscles begin to relax, and the eye movement slows to a roll. Then the body is ready to fulling asleep.

Stage 1

Stage 1 is the first and lightest stage of NREM sleep. The typical characteristics of this stage is the presence of slow eye movements. In this stage people is drowsiness and the body activity reduce nearly 50% than in wakefulness. Muscle half relaxes and brain wave activity begins to slow down. People in this stage can be easily aroused. And if aroused during it, you may feel as if you have not slept yet. Occasionally people may experience hypnic jerks, and someone even have sensation of drifting in and out of Stage 1. Normally stage 1 lasts about 5 to 10 minutes.

Stage 2

Stage 2 is the first actual stage in the defined NREM sleep series. In this stage the body activity reduces continuously. Muscle tone is in spontaneous periods mixed with periods of muscle relaxation. Brain wave activity continues to slow down. And this process may accompany by specific bursts of rapid activity known as sleep spindles and sleep structures known as K complexes, both of which are considered to help  protecting the brain from awakening from sleep. So people in this stage can not be aroused as easily as in Stage 1 sleep. Your heart rate begins to slow and your body temperature begins to decrease. All those clues indicate that your body prepares to enter deep sleep.

Stage 3 and Stage 4

Stage 3 and 4 are known as deep NREM sleep. In many reports these two stages are united into one, because both of them are deep sleep. But there is still some difference between Stage 3 and 4. Generally speaking, people in Stage 4 have deeper sleep than in Stage 3.

Deep sleep is a very critical sleep stage, because most of body recovery is done in this stage. During deep sleep, your body repairs and regrows damaged tissues, builds bones and muscles, and strengthens the immune system. As getting older, you sleep more lightly, which cause you cannot get enough deep sleep, then the body has no sufficient resource to recovery. So you lose muscles and your body becomes weaker and weaker. When getting older, your total sleep time also shortened, although studies have shown that you still need as much sleep as younger.

During deep sleep, it is hard to wake you up, and if did, you would feel slightly dizzy and disoriented for a few minutes. Some people may experience Parasomnias, such as sleepwalking, sleep talking or somniloquy and night terrors during the deepest stage of sleep. In a word, too many things are happened in Stage 3 and 4.

REM Sleep

REM stands for Rapid Eye Movement. From the name, you can see the REM sleep is likely linked to the eye movement. Yes, your eyes move quickly and irregularly from side to side during REM sleep. While in Non-REM sleep, you cannot see such phenomenon. 

You can have intense dreams in REM stage, because your brain becomes more actively than in Stage 2-4 . And your breath and heart rate also quicken. So you might be aroused more easily as well. And if you are awakened, you will fell groggy and sleepy.

Babies can spend up to 50% of their total sleep in the REM stage, with age growing, this number decreases continuously, and for adults there is only about 20% of total sleep in the REM.

You may experience four to five REM periods in a typical night. And they last longer one by one. After the longest REM sleep ends, you wake up.  

In the REM sleep, many body indicators become active again. Brain waves increase to levels when a person is awake. The breath is more rapidly, shallowly and even irregularly, eyes jerk quickly and muscles are temporarily paralyzed. Also, heart rate speeds up, blood pressure rises, and young males even develop erections.

five sleep stages
five sleep stages

What Is Sleep Cycle?

The five stages of sleep occur repeatedly and cyclically. A sleep cycle includes a period of NREM sleep and the followed REM sleep. A period of NREM sleep consists of a series of stage 1-4, and each stage last 5 to 15 minutes. Surprisingly, however, the REM sleep does not follow the Stage 4 directly. Actually, Stage 2, 3 and 4 repeat forward and backward before the REM sleep coming. So a normal sleep cycle looks like this: Wake, Stage 1, 2, 3, 4, 3, 2, REM. Usually, a sleep cycle lasts 90 to 120 minutes. And the first REM sleep comes at 90 minutes after sleep onset and typically lasts 10 minutes. The REM sleep in the next cycle always extends and the final one may last one hour. Commonly you may complete four or five sleep cycles in a typical night.

A Sleep Cycle: Wake ->Stage1 ->Stage2 ->Stage3 ->Stage4 ->Stage3 ->Stage2 ->REM

Different from what you think, one person does not enter REM sleep directly from the deep sleep. In a single sleep cycle you go through from light sleep to deep sleep, then reverse back from deep sleep to light one, finally end with the REM sleep before a new cycle starts over.

Most people begins a sleep cycle with a short period of Stage 1. Though people is easy to be awakened during this stage, Stage 1 is still as important as the following ones.

Stage 2 last longer than Stage 1. And it comprises about 40-60% of total sleep time.

With the sleep cycle going on, you move to Stage 3 and 4 (deep sleep). Commonly deep sleep occupy about 5-15% of total sleep time for adults, which is not as long as Stage 2. For children deep sleep is much higher in duration.

REM occurs at time during the sleep cycle, but on average it begins 90 minutes after sleep onset. The first REM sleep is shortest and only last about 10 minutes, but each of later REM stages gets longer after longer.

sleep stages and sleep cycles percentage
sleep stages and sleep cycles percentage

What Is Deep Sleep And How Dose It Work?

Deep sleep happens in Stage 3 and 4. During those two stages, brain waves appear to be slow speed and large amplitude. We call them delta waves. In many research reports, the Stage 3 and 4 are united into one. Of all of sleep stages, deep sleep is the most restorative and least affected by external stimuli.

It is difficult to wake up a person during deep sleep. But please never think he/she will always lie in bed quietly, actually there may occur many strange actions, such as sleep talking, sleep walking and night terrors, we call these Parasomnias. For baby, bedwetting is common during the deep sleep.

Deep sleep reduces your sleep requirement. If you have a short nap during the day, you can still fall asleep at night as normal. But if you have a long nap and fall into deep sleep, you probably have more difficulty to fall asleep at night, because you have reduced need for sleep.

During deep sleep, you release human growth hormone (HGH) that can stimulates bone growth, cell reproduction, and cell regeneration. So without any treatment, your body recovers and your immune system also restores itself.

REM Sleep and Dream

brain waves during sleep
brain waves during sleep

REM is the time when most vivid dreams occur. The rapid eye movements and low amplitude brain waves  with mixed frequency show higher body activity than in Stages 2, 3 and 4. 

You may dream 4 to 6 times per night, but most of them are forgotten after wake. However, if you are awoken during REM sleep, you can remember the dream often. 

REM sleep is often accompanied by muscle paralysis. This can be interpreted as a protective action to keep one from acting out he/her dreams. Due to the lack of muscle tone, you may experience Obstructive Sleep Apnea, which is the worst thing during REM periods. The breath becomes irregular and shallow, and heart rate and body temperature are also in the irregular status.

Brain Waves During Non-REM And REM Sleep

Sleep science is still a young and frontier domain, and many things need to be studied further. With the new research machines, we have revealed and proved more and more facts in sleep area. Here we list the brain waves in sleep cycles.

STAGE
FREQUENCY (HZ)
AMPLITUDE (MICRO VOLTS) WAVEFORM TYPE
wake 15-50 < 50
pre-sleep 8-12 50alpha rhthym
14-8 50-100 theta
24-15 50-150 splindle waves
31-4 100-200 spindle waves and slow waves
4 1-4 120-200 spindle waves and slow waves
REM 15-30 <50

How Your Sleep Cycle Changes By Age?

In recent years, we often hear a remark, that the only constant in life is ceaseless change. Sleep Cycles also changes throughout your life, especially for deep sleep and REM sleep. For example, the percentage of REM sleep is always highest during infancy and early childhood, then drops off during adolescence and young, and decreases further in older age.

Newborn (0 – 4 months) Do not have distinctive sleep stages.  Sleep is recognized as “Quiet” and “Active”. Quiet sleep is similar to NREM sleep and active sleep is similar to REM sleep. Most of time, newborns are in active sleep and this is necessary for times of feeding.

Infants (4 months – 1 year) Standard sleep stages are apparent now and sleeping routines begin to be developed, sleep is typically 10-13 hours with 2-3 daytime naps.

Toddlers (1 year – 3 years) Sleeping patterns are fully developed, deep sleep occupy about 25% of total sleep time, and it is equal to REM sleep. Spend about 9.5-10.5 hours in sleep per day. Most likely have a nap in the afternoon.

Pre-School (3 – 6 years) Sleep time is about 9-10 hours per day, similar to that of toddlers. The afternoon nap may be unnecessary. Deep sleep still remains high percentage of the total sleep time.

School Age (6 years – 12 years) Sleep 9-10 hours per day and deep sleep remains about 20-25% of total sleep time.  Deep sleep becomes critical for the body growth.

Adolescent (12 years and beyond) Adolescent need about 9-9.5 hours sleep per day. But their circadian rhythm seems a little later. They go to bed later and desire sleep in the morning. As a person ages, the circadian rhythm shifts back and sleep appears to regulate to 6.5-8 hours per day.

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