During sleep, the body goes through multiple sleep cycles. Each cycle consists of four stages: three stages of non-rapid eye movement (non-REM) sleep and one stage of rapid eye movement (REM) sleep.
A person will cycle through the stages of non-REM and REM sleep 4–6 times per night, on average.
In this article, we look at sleep cycle stages, factors that influence them, and how to improve sleep quality.
The sleep cycle is a physiological process that occurs during sleep. It allows the brain and body to perform “housekeeping” functions, such as repairing or growing tissues, removing toxins, and processing memories.
Each sleep cycle consists of four stages, with each having varying effects on the body. On average, adults go through 4–6 sleep cycles per night and spend 90 minutes in each sleep cycle stage.
Below, we list the four stages of the sleep cycle.
Stage one begins when a person shifts from wakefulness to sleep. It is a period of light non-REM sleep that slows down a person’s heart rate, breathing, eye movements, and brain waves. The muscles also relax, although they may twitch occasionally.
This stage is short and lasts for around 1–5 minutes.
This is a period of deeper non-REM sleep, where the muscles relax further, eye movements stop, and body temperature drops.
Stage 3 non-REM sleep is the deepest stage of sleep and the hardest to awaken from. During this stage, heart rate, breathing, and brain waves become regular.
A person will experience the most deep sleep during the first half of the night. With each sleep cycle, the amount of deep sleep decreases.
This is the stage people typically find most difficult to wake from. If a person wakes during deep sleep, they may feel mentally foggy for around 30–60 minutes. The overall percentage of deep sleep tends to decrease with age.
The last stage of the sleep cycle is REM sleep. The term “REM” refers to a person’s eye movements. During this stage, the eyes move quickly and rapidly from side to side.
During REM sleep, breathing quickens and becomes more erratic. Other vital signs, such as blood pressure and heart rate, become less regular.
REM is the sleep stage most associated with dreaming, although dreaming can also occur in other stages. During this time, most people experience muscle atonia, or temporary muscle paralysis, which occurs naturally during REM sleep and prevents a person from acting out their dreams.
Rarely, the loss of muscle tone usually associated with REM sleep may not occur. This condition is known as REM sleep behavior disorder.
REM sleep lasts for approximately 10 minutes during the first sleep cycle, increasing in length as the night progresses. In the final cycle of sleep, REM can last up to 1 hour.
There are numerous factors that can have an impact on the length and quality of the sleep cycle and the amount of time people spend in each stage. These include:
Adults typically fall asleep through non-REM sleep, while infants fall asleep through REM. Infants spend a much greater part of the night in REM sleep compared with adults.
The percentage of deep sleep is higher in children than in adults, and it decreases with age. Most sleepwalking episodes arise out of deep sleep, which is why sleepwalking is more common in children.
Some medications have significant effects on sleep. For example, benzodiazepines decrease the amount of time a person spends in the deep sleep and REM sleep stages.
Caffeine can decrease the amount of deep sleep and REM sleep a person gets, lowering sleep quality.
According to a 2013 review, while alcohol can help some people fall asleep, it also fragments sleep later in the night and reduces sleep quality.
Some health conditions disturb the usual progression of sleep cycle stages. For example, narcolepsy — which causes daytime sleepiness and muscle weakness — often causes people to go straight into REM sleep, skipping the first three stages.
People with sleep apnea may struggle to get as much deep sleep and REM sleep due to breathing difficulties.
Stress and mental health conditions
Stress and anxiety can cause sleep fragmentation. This may be the body’s way of preparing for danger, making it easier for a person to wake up.
The amount of sleep people need varies depending on their age. Sleep duration requirements may also vary slightly from person to person.
The Centers for Disease Control and Prevention (CDC) provide the following recommendations for sleep duration by age group.
|Age||Total sleep amount per day|
|0–3 months||14–17 hours|
|4–12 months||12–16 hours, including naps|
|1–2 years||11–14 hours, including naps|
|3–5 years||10–13 hours, including naps|
|6–12 years||9–12 hours|
|13–18 years||8–10 hours|
|18–61 years||7 hours or more per night|
|61–64 years||7–9 hours|
If a person notices that they do not feel refreshed when they wake up in the morning or that they experience frequent tiredness during the day, they may not be getting good quality sleep.
This can occur even if the person sleeps for a proper amount of time each night.
In many cases, sleep quality can improve with lifestyle changes. These include:
- developing or maintaining a regular schedule for sleeping and waking, including on weekends
- using the bed or bedroom only for sleep and sex, if possible
- reducing caffeine and alcohol consumption, particularly in the evening
- getting exposure to natural light during daytime or using a light therapy device
- reducing exposure to blue light, such as in phones, computers, and other devices, before bed
- making time each evening to rest and relax, avoiding sources of stress
Low sleep quality can also be a side effect of certain medications, drug or alcohol overuse, mental health conditions, hormone imbalances, and sleep disorders. If any of these is a concern, it is best to speak to a doctor.
If a person suspects they are not sleeping well and if lifestyle changes do not help, they may benefit from speaking to a doctor.
A doctor can help assess a person’s symptoms and determine whether they have a sleep disorder or another underlying condition that could be lowering their sleep quality.
Possible symptoms of a sleep disorder include:
- chronic fatigue
- daytime sleepiness
- irritability and mood changes
- trouble concentrating or making decisions
- sleepwalking or sleep talking
- teeth grinding or jaw clenching
- depression or anxiety
There are four sleep cycle stages, beginning with light sleep and ending with REM sleep. Each stage has a different effect on the body, and each is important for sleep quality.
Many factors can alter the stages of sleep, including stress, caffeine, and certain medications. What helps improve sleep quality will vary from person to person.
In some cases, lower quality sleep may result from aging, while in others, it may stem from a sleep disorder. If a person has tried various methods for improving their sleep and does not find any relief, they can consider seeking help from a doctor or sleep specialist.