A night’s sleep is made up of sleep cycles. There are four individual stages of a sleep cycle over the course of a night.
Typically, a person has four-to-six sleep cycles in a night that last for approximately 90 minutes each.
Stages of Sleep
There are four sleep stages in a sleep cycle. The first three stages are non-rapid eye movement or non-REM sleep (NREM Sleep) and the fourth is rapid eye movement (REM sleep). What differentiates each stage is an individual’s brain activity during sleep. Distinct sleep patterns determine the stage a person is in.
The stages of sleep are listed below, as well as the lengths of all four stages.
Stage 1: NREM Sleep (N1) – 1-5 minutes
Stage 2: NREM Sleep (N2) – 10-60 minutes
Stage 3: NREM Sleep (N3, Slow-wave sleep, Delta Sleep, Deep Sleep) – 20-40 minutes
Stage 4: REM Sleep – 10-60 minutes
The higher the NREM stage, the deeper the sleep. NREM sleep is made up of three different stages: N1, N2 and N3, then REM Sleep.
Stage 1: NREM Sleep (N1)
When a person begins to go to sleep, they’ve entered the N1 stage. The first stage is a light sleep in which the individual can easily be woken up, but it only lasts four approximately 1 to 5 minutes before Stage 2.
N1 sleep is the stage during which brain activities begin to slow down and can cause your body to twitch and move slightly.
Stage 2: NREM Sleep (N2)
The body enters a deeper sleep in stage 2. Body temperature drops, there is relaxed muscle activity, and also a slower heart rate. Brain waves show a different pattern than stage 1 and brain activity continues to slow down.
The N2 stage of sleep lasts for approximately 10-25 minutes during the first sleep cycle but can last longer as the night goes on. About 50 percent of your sleep time is spent in N2 NREM Sleep.
Stage 3: NREM Sleep (N3)
The N3 stage is when a person has entered a deep sleep and it is difficult to wake them up.
Stage 3 sleep is also known as deep sleep, and it is harder to wake someone up if they are in this phase. Muscle tone, pulse, and breathing rate decrease in N3 sleep as the body relax even further.
The brain activity during this period has an identifiable pattern of what is known as delta waves. For this reason, stage 3 may also be called delta sleep or short-wave sleep (SWS).
Experts believe that this stage is critical to restorative sleep, allowing for bodily recovery and growth. It may also bolster the immune system and other key bodily processes. Even though brain activity is reduced, there is evidence that deep sleep contributes to insightful thinking6, creativity7, and memory.
We spend the most time in deep sleep during the first half of the night. During the early sleep cycles, N3 stages commonly last for 20-40 minutes. As you continue sleeping, these stages get shorter, and more time gets spent in REM sleep instead.
Stage 4: REM Sleep
Rapid eye movement or REM Sleep is the fourth and final stage of the sleep cycle. Brain activity picks up in the REM stage and while the eyes are closed, they move rapidly. There is a temporary paralysis of muscles other than the eyes and those that control breathing. Many experience vivid dreams during this stage.
REM Sleep is considered integral to cognitive functions such as memory and learning. It usually takes approximately 90 minutes to enter the REM sleep stage. REM periods typically get longer as the night goes on.
What can affect sleep patterns?
Sleep stages tend to stick to a typical pattern, but there are a number of variables that could affect one’s sleep patterns during a night’s sleep. Here are a few:
- Age: Over the course of a person’s life, their time in each sleep stage changes. For example, babies typically spend half of their night’s sleep in REM sleep. That time decreases to that of an adult’s (about 25 percent) as they get older. Elderly individuals spend less time in REM sleep.
- Alcohol consumption: Sleep patterns can be significantly impacted by one’s alcohol and drug intake. If someone consumes alcohol before bed, they likely will have less REM sleep and thus may not have a good night’s sleep.
- Sleep disorders: Conditions such as Sleep apnea and Restless Leg Syndrome can interrupt one’s sleep cycle.
Keys to better sleep
There are steps a person can take to ensuring a healthy sleep cycle. Perhaps the most important of all is to keep a consistent sleep schedule with about 6 to 8 hours of sleep a night. Sleep deprivation will lead to poor quality of sleep, as well as sleepiness during the daytime.
Other ways to improve one’s sleep include avoiding alcohol, eliminating noise and light, and avoiding negative and anxious thoughts. These improvements can lead to uninterrupted sleep and help correct the alignment of your circadian rhythm. If you get enough sleep, and these sleep habits do not improve your daytime sleepiness, or you believe you may have a sleep disorder, you should speak with a doctor about how to proceed.