When considering getting the sleep you need, it is common to focus on how many hours of sleep you are getting. While the amount of sleep is undoubtedly important, it is not the only part of the equation.
It is also important to consider the quality of sleep and whether the amount of sleep is truly restorative. Moving smoothly through multiple sleep cycles consisting of four separate sleep stages is an important part of getting true quality rest.
What is sleep cycles1? On a typical night, a person will experience four to six sleep cycles. Not all sleep cycles are equally long, but on average, each sleep cycle lasts about 90 minutes.
The sleep cycle is usually composed of 4 stages which is considered by the activities of brain.
Sleep cycles often consist of REM and NREM. As shown in the figure below, the N1, N2 and N3 stages are collectively referred to as NREM sleep stages.
NREM sleep
NREM sleep consists of three distinct stages. the higher the NREM sleep stage, the more difficult it is to wake a person from sleep.
N1
N1 stage is essentially a "dozing" stage that usually lasts only one to five minutes.
During N1 sleep, the body is not fully relaxed, although body and brain activity begins to slow down with brief movements (twitches). There is a slight change in brain activity associated with falling asleep during this stage.
It is easy to wake a person during this stage of sleep, but if a person is not disturbed, they can move quickly into stage 2.
N2
In stage 2, the body enters a gentler state that includes a drop in temperature, muscle relaxation, and slower breathing and heart rate. At the same time, brain waves show a new pattern and eye movements stop. Overall, brain activity slows, but there are brief bursts of activity.5 This actually helps to resist being awakened by external stimuli.
Stage 2 sleep can last 10-25 minutes in the first sleep cycle, and each N2 stage becomes longer during the night. Overall, a person typically spends about half of their sleep time in N2 sleep.
N3
N3 sleep stage, also known as deep sleep, makes it more difficult to wake someone up if they are in this stage. As the body relaxes further, muscle tone, pulse rate and breathing rate decrease during N3 sleep.
Experts believe that this stage is essential for restorative sleep, allowing the body to recover and grow. It also enhances the immune system and other key bodily processes. N3 stage usually lasts 20-40 minutes. As you continue to sleep, these stages become shorter, and more time is spent in REM sleep.
REM sleep
REM sleep is known for the most vivid dreams, which can be explained by a significant increase in brain activity. Dreams can occur in any stage of sleep, but are less common and intense in non-REM periods.
REM sleep is considered essential for cognitive functions8 such as memory, learning and creativity3.
Under normal circumstances, you do not enter the REM sleep stage until you have slept for about 90 minutes. As night falls, the REM phase becomes longer, especially in the latter part of the night. While the first REM stage may last only a few minutes, the later stages may last about an hour. Overall, the REM stage accounts for about 25% of adult sleep.
Sleep stages are important because they allow the brain and body to recover and develop. Failure to get enough deep sleep and rapid eye movement sleep can explain some of the profound effects of sleep deprivation on thinking10, mood and physical health.
Sleepers who wake frequently in the early stages, such as those with sleep apnea, may have difficulty cycling properly into these deeper stages of sleep.
Resources:
1.Patel, A. K., Reddy, V., & Araujo, J. F. (2020, April). Physiology, Sleep Stages. StatPearls Publishing. Retrieved from
https://www.ncbi.nlm.nih.gov/books/NBK526132/
2.Division of Sleep Medicine at Harvard Medical School. (2007, December 18). Natural Patterns of Sleep. Retrieved July 28, 2020, from
http://healthysleep.med.harvard.edu/healthy/science/what/sleep-patterns-rem-nrem
3. Stages of Sleep. Eric Suni, Dr. Nilong Vyas, https://www.sleepfoundation.org/stages-of-sleep
