How Sleep Cycles Work: a Guide to Stages of Sleep

by | Oct 16, 2019

Reading Time: 6 minutes

Did you know, that sleep is a much more active state of the body that many people would think?

During a regular sleep, your brain goes through various phases of activity, which are also called a sleep cycle. Main stages of a sleep cycle include non-REM (NREM) sleep (4 stages) and then REM or rapid eye movement stage.

The main purpose of sleep is for your organism to recover from the previous day and therefore a good quality sleep is super important for your health.

So, let’s explore how sleep cycles work by delivering a complete guide on the different stages of sleep.


What is NREM Sleep?

Non-REM sleep has three or four distinct stages that you go through before getting to REM sleep. These stages have been studied using electroencephalogram (EEG), which shows the difference in brain activity in each of the stages.


NREM Stage 1

This stage happens within minutes of drifting off to sleep. During this stage sleep is light and it is easy to wake up. The brain produces alpha and high amplitude theta waves, which slow it down. Stage 1 is, in fact, the transition period between being awake and sleep, and it only lasts for a brief period of about 5 to 10 minutes.

During this stage, some people may experience a falling sensation or hear someone calling their name. Others might experience myoclonic jerk, which is characterized by being startled suddenly for no reason. This phenomenon might seem unusual but they are quite common.


NREM Stage 2

Stage 2 can be defined as the first stage of true sleep. The sleep is still light, but you become less aware of your surroundings, your brain wave activity slows down and body temperature drops.

Stage 2 lasts for about 20 minutes. During this stage, you will have sleep spindles which are sudden rapid bursts of brain wave activity. On average, this is the stage where people spend an approximate 50 per cent of their total sleep.


NREM Stage 3

Stage 3 is deep sleep. Previously, what is now known as stage 3 was split into two stages, stage 3 and stage 4, depending on the frequency of delta waves. Stage 4 was initially characterized by delta waves more than 50 per cent of the total. The brain starts producing slower delta waves, that’s why this stage is sometimes also referred to as delta sleep. In this phase, your body becomes less responsive to the surroundings and it becomes harder for you to be awakened. Dreaming is more common during this stage than in any other non-REM sleep phases, but not as common as during the REM sleep.

It is becoming more difficult to wake a person during this period. If a person is awakened at this stage, they will often feel disoriented and it may even take around 30 minutes before they get back to normal brain performance. That is commonly known as sleep inertia.

Stage 3 occurs in longer durations during the first half of the night and especially during the first two sleep cycles. Adults spend about 20 percent of their total sleep in this stage. The brain temperature, heart rate, breathing rate and blood pressure are all at their lowest.

During this stage, your body can also be termed to be in a restorative stage of sleep. The body repairs and regrows tissues build bone muscles and strengthen the immune system. There are older studies that suggest that bed-wetting is more likely to happen during this stage of sleep. Also, sleepwalking occurs most often during this stage of deep sleep.


What is REM Sleep?

REM is rapid eye movement sleep. It is the final stage of sleep, during which the most dreaming occurs. The first REM stage lasts for about 10 minutes while the others that follow can last for up to one hour. During the final phase of the sleep cycle, your brain becomes more active, your muscles are relaxed and you have more dreams. Your eyes move quickly in different directions, your heart rate and blood pressure increase and you experience shallow and irregular breaths.

REM sleep is the most important phase of sleep for learning and memory function. This is so because your brain is consolidating and processing information from the previous day which gets stored for long term memory.

REM sleep is also sometimes referred to as paradoxical sleep because while the brain becomes more active, your muscles become more relaxed and your body becomes immobilized.

The length of REM stages can last for different durations, but the periods increase as the night progresses. That’s why you’re likely to wake up from a dream in the morning. The American Sleep Foundation suggests that adults spend up to 20 per cent of their sleep in the REM stage compared to 50% for babies.


Sleep Architecture

Sleep architecture is the pattern of sleep that we each get and the amount of time we spend in each stage. Most adults require about 7 hours of sleep every night, while teenagers may require about 9 hours of sleep due to their developing brains. Babies, on the other hand, have the fastest developing brains of all groups, and therefore they need up to 16 hours of sleep every day.

Each sleep cycle lasts for about 90 minutes and is repeated four to five times throughout the night. The early stages of the sleep last for shorter periods.

Sleep architecture varies at different ages. Babies can spend approximately 50 per cent of their sleep in REM compared to only 20 per cent for adults. Children, on the other hand, have longer deep NREM sleep. The elderly people have a reduced deep sleep, but their sleep in stages 1 and 2 increase. Elderly people have easily disturbed sleep.


Cleaning the Brain During Sleep

In 2014, Jeff Iliff, a famous neuroscience researcher presented his speech on TED on how the brain clears its “waste.” His research indicated that when we sleep, there are channels that open up around the blood vessels in the brain. These channels allow the cerebrospinal fluid (CSF) to flush out, which removes the waste products.

Watch the full video below:



Waking Up in the Middle of a Sleep Cycle

Waking up in the middle of a sleep cycle may give you a disoriented feeling, especially during stage three or later. You need to finish the duration for each stage for you to get benefits of sleep. Going for at least 90 minutes until you’re back at stage one is the best way to have a good night sleep.

Note, that high quality sleep is not about the amount of sleep you get. It is about the number of complete sleep cycles you go through over the night. Sleep cycles are repeated four to five times over the course of the night. Therefore, if you happen to finish four cycles after about six hours you may feel more relaxed than waking up after 8 hours but in the middle of a REM cycle.

Complete the number of cycles recommended every night is the secret to waking up feeling more rested and ready for the day. Therefore, try to optimize your sleep schedule in order to avoid waking up in the middle of a sleep cycles.

Some animals are more likely to wake up after each cycle of REM sleep than most people. However, there are people who are more likely to wake up from REM sleep than from non-REM sleep. Usually, these types of awakening last for a few seconds only and one might normally not remember them. But if the person is highly stimulated, he or she may wake up fully, and it may take the length of a whole sleep cycle (about 90 minutes) to get back to sleep.


What Impact Lack of Sleep Might Have?

Having not enough or poor quality sleep lowers all the positive things that happen while you are asleep. Your body is unable to rest and restore effectively, biological processes get disturbed and your brain function and cognition could be negatively affected.

Here are some consequences of the poor quality sleep:

  • Cognitive performance: lack of sleep affects your cognitive performance. Your brain will experience reduced focus and slower processing of information which can lead to impulsivity and poor decision-making.
  • Immune system: lack of sleep may lead to a weakening of the immune system, thus you become more prone to diseases.
  • Heart health: while you are asleep your heart takes a rest. Inadequate or poor quality sleep can lead to increased strain on your heart.
  • Memory: when you are asleep your brain is able to consolidate memory and process information and store it for the long-term. Lack of sleep limits the ability of the brain to consolidate memory.

Hormonal systems: lack of sleep negatively affects the hormonal systems which produced chemicals needed by every cell, system or organ in your body. Lack of sleep also affects cellular repair and growth, blood pressure, blood sugar and sexual health.

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