The Four Stages Of Sleep And Sleep Cycles

Last updated: March 6, 2019

Overview

After we fall asleep, our body goes through different stages. These stages then repeat cyclically, and each of them is characterized by certain brain waves, body temperature, responsiveness, ability to dream. Stages of sleep and sleep cycles were first discovered and recorded after the invention of EEG (electroencephalograph), which was used to detect brain waves in sleeping people.

What are REM and Non-REM sleep?

REM (rapid eye movement) refers to the stage of sleep in which we have the most vivid dreams. Non-REM (NREM) sleep is any of three other stages in which our eyes are completely restful and dreams may happen, however, not so vividly and frequently (it was previously believed that dreams don’t happen in NREM). Stages of NREM sleep are divided into light and deep sleep.

REM Sleep chart
The four stages of sleep.

What are the four stages of sleep?

The two stages of light sleep are Stage 1 and Stage 2. As they occur during NREM sleep, they are also called N1 and N2.

Stage 1 NREM

It is very easy to wake up from Stage 1 sleep because it is the closest stage to being awake. People will usually deny being asleep if woken up from this stage, as they are still aware of their surroundings. This stage lasts between 1 and 10 minutes.

Stage 1 characteristics:

  • Breathing slows down
  • Heart rate becomes regular, blood pressure drops
  • Body temperature drops
  • The feeling of falling down and waking up suddenly – the hypnic jerk
  • The brain produces alpha and theta waves

Stage 2 NREM

Stage 2 is also a light sleep stage – however, it prepares our body for deeper stages of sleep and lasts up to 20 minutes. Sleepers in this stage are harder to wake up.

Stage 2 characteristics:

  • Muscles are relaxed
  • Metabolic processes slow down
  • The brain produces waves of high-frequency bursts known as sleep spindles
  • People spend about 50% of their total sleep time in this sleep phase

Stage 3 NREM

Deep sleep equals Stage 3 (N3). This used to be separated into Stages 3 and 4, but has recently been merged into one stage. N3 stage of NREM sleep is also known as delta sleep – the name comes after the type of waves the brain emits. It is very difficult to wake up a person from the deep sleep. N3 stage is known to be a phase of restorative sleep. This stage begins about 35-45 minutes after falling asleep.

Stage 3 characteristics:

  • Human growth hormone is released, promoting cell repair and body restoration
  • It is believed that this stage is responsible for psychological refreshment, preparing the brain for new learning
  • If woken up from this stage, a person may feel sluggish and disoriented
  • Certain drugs, alcohol, excessive stress, aging or sleep apnea can prevent us from reaching deep sleep
  • There is no muscle activity

Stage 4 REM

Stage 4, or the final stage of a sleep cycle is the REM stage. Rapid eye movement sleep occurs after about 90 minutes of sleeping; the first REM lasts for about 10 minutes whereas it gets longer in the second half of the night. Very young children spend more time in REM sleep than adults.

REM sleep characteristics:

  • Brain activity increases, muscles remain relaxed
  • Brain waves are similar to those during wakefulness
  • Breathing becomes irregular and shallow
  • Heart rate increases
  • Vivid dreams happen
  • Eyes move rapidly, probably following the dream scenarios
  • The brain processes the information from the day before to store it in the long-term memory

Another term for REM sleep is paradoxical sleep because even though brain activity is on the high, muscles remain paralyzed. This may be a mechanism which protects us from getting hurt by shutting down muscles so that they can’t move along with the dreams. Interestingly enough, our eyes are not hindered.

Sleep cycles are sequences of sleep stages

Sleep stages don’t occur in a simple 1, 2, 3, 4, 1 sequence. They are lined up like this (blue – light sleep, orange – deep sleep, red – REM):

Stage 1 – Stage 2 – Stage 3 – Stage 2 – REM – Stage 2

From this cycle, we can easily understand how come people spend most of their sleep time in Stage 2. It is also logical that Stage 3 may be completely omitted – REM sleep is preceded and followed by NREM Stage 2. Therefore, we can sleep and dream without having the much-needed restorative Stage 3 deep sleep. Stage 1 only appears after a period of wakefulness.

During one night’s sleep, there are about 4-6 sleep cycles (the first of which lasts about 90 minutes, and the rest a bit longer, about 110-120 minutes). However, not every REM cycle contains sleep stages of the same duration. This means that in the first half of the night we will spend most of the time in Stages 2 and 3, whereas in the second half REM stage dominates. One period of REM sleep may last between 10 minutes and an hour.

Talking about the first and second half of the night, there is one phenomenon which hasn’t yet been fully understood or explained. During the early hours of the night, we experience more NREM sleep, that is, between 11 pm and 3 am, whereas between 3 and 7 am we experience more REM sleep. This is true even for those who go to bed at 3 am – they will only have more REM sleep, even though they haven’t gone through the process of the dominant NREM sleep.

Even though it is not clear why this is so, what we can do about it is make sure we don’t stay up later than 11 pm, so that we can get more of rejuvenating deep sleep. What matters is not only the total number of hours of sleep, it is also when we go to sleep that determines our sleep quality.

How to get the best out of your sleep?

Stick to the same schedule every day of the week, with respect to your chronotype. Always go to bed and wake up at approximately the same time, including the weekends. This will get your biological clock in a good working order. Depriving yourself of sleep in the week and trying to make up for it at the weekend will not do you a favor – you can’t get out of the sleep debt so easily. Such behavior causes social jet lag and sleep deprivation. Keeping your sleep-wake cycle to a regular schedule may help prevent some sleep disorders associated with sleep deprivation.

Skip the snooze button. By sticking to the same time schedule, your body will learn what your waking time is and will adjust sleep stages so that you wake up during a light sleep stage. Hitting snooze button over and over will confuse your circadian rhythm and you will have trouble waking up. Remember, although it will be difficult to get rid of snooze at first, the reward you’ll enjoy later will be worth it.

Keep your daytime naps short. During a short daytime nap, you will be well rested for the day and not have problems having a good night’s sleep. This is because during a nap of about 20 minutes your brain will remain in Stages 1 and 2 – the ones you can easily wake up from. If you have a long nap, you run the risk of your alarm waking you up mid-deep sleep where you’ll feel even more tired. If you take a 90-minute nap, which is enough to go through the whole cycle, you will most likely get some deep sleep – this is good for your rest, but will lessen your sleep needs, which means going to bed late that night and disrupting your sleep patterns.

Additional Resources

  1. Dreaming also occurs during non-rapid eye movement sleep. Science Daily. August 9, 2016. https://www.sciencedaily.com/releases/2016/08/160809121817.htm Accessed on December 4, 2018
  2. The 4 Different Stages of Sleep. Withings. Posted on March 17, 2015. https://blog.withings.com/2015/03/17/the-4-different-stages-of-sleep/ Accessed on December 2, 2018
  3. Cline J. The Mysterious Benefits of Deep Sleep. Psychology Today. Posted on October 11, 2010. https://www.psychologytoday.com/us/blog/sleepless-in-america/201010/the-mysterious-benefits-deep-sleep Accessed on December 2, 2018
  4. Gordon A. M. Your Sleep Cycle Revealed. Psychology Today. Posted on July 26, 2013. https://www.psychologytoday.com/us/blog/between-you-and-me/201307/your-sleep-cycle-revealed Accessed on December 2, 2018
  5. Chronotypes – What They Are and What You Can Do About Them. Sleep Kid. Posted on November 30, 2018, https://www.sleepkid.com/chronotypes/ Accessed on December 2, 2018
  6. Sleep Debt – Can You Make Up Lost Sleep? Sleep Kid. Posted on November 14, 2018, https://www.sleepkid.com/chronotypes/ Accessed on December 2, 2018
  7. Sleep Deprivation – How Losing Sleep Can Ruin Your Health. Sleep Kid. Posted on November 14, 2018, https://www.sleepkid.com/chronotypes/ Accessed on December 2, 2018
  8. Circadian Rhythm and Sleep. Sleep Kid. Posted on November 14, 2018, https://www.sleepkid.com/chronotypes/ Accessed on December 2, 2018.

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