What is a sleep cycle? Most of us know that we have several stages of sleep, but it's important to have a good understanding of the different stages of sleep and the difference between REM sleep and NREM sleep, as well as the basics of sleep cycles.
What is a Sleep Cycle?
As a person sleeps, their body rests but the brain remains active. This allows the brain to perform a number of different functions, including consolidating memory and improving how our neurons communicate with each other. Some researchers say sleep serves as a chance for our brain to remove toxins. This is why sleep is considered as essential as food and water for the human condition.
There are two basic types of sleep in a sleep cycle: Rapid Eye Movement (REM) and Non-Rapid Eye Movement (NREM). During NREM sleep, there are three separate stages of sleep. The amount of sleep occurring during each of these stages changes throughout a person's life, particularly as a person ages. Each stage, including REM, affects the brain in different ways, and sleep cycles between REM and NREM sleep several times a night depending on how long one sleeps and the quality of that sleep.
Typically, sleep begins with a NREM sleep stage, cycles through the three NREM stages, and is followed by a REM period. Throughout the night, NREM and REM sleep alternate in a cyclical fashion, over approximately 90 minutes with REM sleep periods getting progressively longer. A sleep cycle can average 70 to 120 minutes, with an average of three to four cycles occurring over a night of sleep.
REM sleep consists of about 20%-25% of total sleep in adults. From a scientific perspective, REM sleep is initiated through acetylcholine secretion and inhibited by neurons that secrete monoamines including serotonin. During REM, most muscles experience temporary paralysis. This is also the stage during which people experience dreams. Interestingly, the percentage of REM sleep in adults changes little over the lifespan, while slow-wave sleep tends to decline with age.
An adult experiences REM sleep every 90 minutes or so, and studies show brain activity is at its highest level at this time. Typically, the longest REM period occurs at the end of a night's sleep and is cut short if a person does not get their full night of sleep.
No one knows exactly why we have REM sleep, but research shows this stage is essential in keeping our brains sharp. Several studies have explored the benefits of REM sleep and found that not only does an increase in REM improve depression, anxiety and cognitive brain function, it also decreases blood pressure and provides other overall health benefits.
While REM deprivation can be detrimental to health, there is evidence showing the body can recover from a lack of it. REM sleep rebound is an increase in REM sleep above normal levels after a period of sleep restriction or deprivation. This can be especially important to understand, given many with obstructive sleep apnea have this particular stage of their sleep affected, and may have REM rebound when their sleep disordered breathing is treated.
Non-REM (NREM) sleep uses significantly less energy than REM sleep. This type of sleep is associated with the brain restoring its supply of adenosine triphosphate (ATP). NREM sleep is divided into three separate sub-stages: N1, N2 and N3 or slow-wave sleep. Each of these stages can last from 5 to 15 minutes or more and NREM stages may repeat until REM sleep is attained.
NREM Stage 1 (N1)
This stage lasts about 5 to 10 minutes and is the transition period between wakefulness and sleep. This stage can also occur during transitions between periods of deep sleep and REM as well. The muscles in the body remain active and a person in this stage of NREM sleep can be easily awoken.
NREM Stage 2 (N2)
This stage is a period of light sleep with slowing brain waves and muscle relaxation. It is generally the period between drowsiness and light sleep and deep slow wave sleep. Adults spend about 50% of their entire night's sleep in stage 2 sleep. Toward the end of a period of stage 2 sleep, as the body prepares to enter deep sleep, the heart rate slows and body temperature decreases.
NREM Stage 3 (N3)
Stage N3, slow-wave sleep, assists the body to repair and regenerate tissues. A recent study suggests this is also the stage during which the brain flushes out toxic waste. According to a study from Boston University, research shows cerebrospinal fluid may clean out toxic waste from the brain, and that slow- wave sleep greatly aids in the process. This stage of sleep may last up to 40 minutes.
As we age, our sleep needs change. This is precisely why there are different sleep recommendations for different age groups. According to the National Sleep Foundation (NSF), each age group has its own set of recommended sleep time ranges for optimal sleep health. As you can see based on these recommendations, sleep time is predominant in infants, and as we age, our sleep needs tend to decrease.
- Newborns (0-3 months): 14-17 hours a day
- Infants (4-11 months): 12-15 hours a day
- Toddlers (1-2 years): 11-14 hours a day
- Preschoolers (3-5 years): 10-13 hours a day
- School-Aged Children (6-13 years): 9-11 hours a day
- Teenagers (14-17 years): 8-10 hours a day
- Younger Adults (18-25 years): 7-9 hours a day
- Adults (26-64 years): 7-9 hours a day
- Older Adults (65+ years) 7-8 hours a day
These NSF guidelines relating to sleep duration are recommended to improve the health, performance and safety of people of all ages.