Sleep Stages: Understanding Your Sleep Cycles
November 19, 2019 | Casper Editorial Team

Many people believe the brain “shuts off” to rest during sleep. However, that’s not necessarily the case — there’s more going on behind the eyelids than one may think. The brain goes through several different cycles throughout the time it’s sleeping.
These cycles evolve through the various stages of sleep, starting with NREM (Non-Rapid Eye Movement) sleep and progressing to REM (Rapid Eye Movement) sleep. The typical person starts a new sleep cycle every 90 to 120 minutes, meaning they likely go through four or five cycles during a night of rest.
Here we’ll cover the different stages of sleep, which are the most important, how much sleep one should get and how sleep changes throughout a lifetime.
A graph showing the different sleep cycles: Stages 1-3: NREM Sleep, lasts around 4 to 7 hours per night. Stage R: REM Sleep, lasts 90-120 minutes per night.

What Are the Stages of Sleep?

There are four stages of sleep — three NREM stages and one REM stage. The stages start out very light and progress to deeper sleep. When a sleep cycle finishes, the person will reverse from REM to less deep stages, ending up again in the lightest phase of sleep. Then the whole cycle begins again.
Stage 1 
The sleep cycle begins with stage 1 sleep. This is when the body starts to relax and become drowsy. This phase is often identified by a person’s slow, rolling eye movements and ability to be easily awakened.
At this time, brainwave activity begins to slow down to theta brain waves as the body drifts to sleep. Some people may experience sudden jerks or muscle spasms during this phase or a sensation of falling. If the person is woken up during this time, they’re likely to think they weren’t asleep at all. Stage 1 usually lasts about 10 minutes.
A person snoozes with their legs crossed on a Casper mattress. Illustration.
Stage 2 
The first phase where the person is fully asleep is stage 2, which tends to occur for longer periods than stage 1. A person isn’t as easily roused during this time and their slow-moving eye rolls typically stop. Their body temperature will begin to drop and their heartbeat will slow.
A person’s brain waves will continue to slow as they drift deeper into sleep. However, during this phase, there are also bursts of rapid activity known as sleep spindles. After a spindle occurs, the brain waves slow down again. Typically, stage 2 makes up 40 to 60 percent of a person’s total sleep time.
Gentle waves coming from a person's brain. Illustration.
Stage 3 
As the body begins to move through sleep, it will enter stage 3. This is a restorative stage and doesn’t typically last as long as stage 2. It makes up five to 15 percent of total sleep time for most adults. However, children and teenagers will typically spend much longer in this stage.
In this deep sleep, a person’s brain waves slow down to become delta waves as the body prepares for REM sleep. During this time, the body strengthens its immune system, repairs and regrows tissues, and builds bone and muscle. This is also when parasomnias like sleepwalking, talking or night terrors happen.
During stage 3 sleep, it’s much harder for the body to wake — if a person does wake from it, they’ll likely feel disoriented at first. As the body produces more delta brain waves in this stage, it drifts closer to REM sleep.
Construction cranes work on repairing a giant human body. Illustration.
Stage R (REM Sleep) 
Phase four is called stage R, or REM sleep, and is the deepest of the four stages. This is when people usually experience the most vivid dreams. The first REM phase of the night is typically shorter and begins around 90 minutes after a person begins to fall asleep, lasting about 10 minutes.
Throughout the night, a person will experience longer periods of REM as they complete more sleep cycles ending at between 70 to 100 minutes. The average adult has five or six REM cycles in one sleeping period.
During this time, the brain becomes more active. The person’s eyes will often jerk quickly in different directions, their blood pressure and heart rate will increase, and their breathing may become irregular. Their arm and leg muscles become paralyzed so that they don’t act out their dreams.
REM sleep helps the brain consolidate and process information from the previous day and helps the brain store this information in a person’s long-term memory. As a person gets older, they spend less time in REM sleep.
A graph showing light and deep sleep cycles. Over an 8-hour period, the body fluctuates between light sleep and deep sleep every hour or so.

What Are The Most Important Stages of Sleep?

Not all sleep is created equal — some stages are more important than others. The work that happens during stage 3 and REM sleep is different but equally important. Both of these stages are considered deep sleep, which is important for growth, hormone regulation and physical renewal.
In REM sleep, the brain processes memories and emotions, allowing humans to have higher-level learning and thoughts.
If a person were to forgo deep sleep, they would likely feel sick and depressed, and even gain weight. Their cognitive processes would begin to slow and they may have difficulty concentrating and socializing.
A sleeping brain filtering happy and sad faces into separate boxes. Illustration.

When Does REM Sleep Occur?

REM sleep usually happens around 90 minutes after one falls asleep and the first session lasts about 10 minutes. As the person continues to cycle through sleep, the REM stages get longer, with the final stage lasting around one hour.
A pair of eyes going from fully open, to half open, to closed. Illustration.

How Much Deep Sleep Should You Get?

Studies show that the average adult needs anywhere from 1.6 to 2.25 hours of deep sleep per night. Since deep sleep is so important for brain health and function, it’s imperative that each person spends 20 to 25 percent of their sleep in this state.
Children and babies need more time in deep sleep since their bodies are still growing. Kids up to two years of age need around 4.5 hours of deep sleep. Those between two and 12 years need a bit less — around four hours. Teenagers and young adults need about three hours of deep sleep.
A graph outlining sleep requirements at different ages as follows: Newborn to 3 months: 12-18 hours of sleep, 2.4 to 3.6 hours of deep sleep, 3 months to 1 year old: 14 to 15 hours of sleep and 2.8 to 3.0 hours of deep sleep, 1 to 3 years old: 12 to 14 hours of sleep and 2.4 to 2.8 hours of deep sleep, 3 to 5 years old: 11 to 13 hours of sleep and 2.2 to 2.6 hours of deep sleep, 5 to 12 years old: 10 to 11 hours of sleep and 2 to 2.2 hours of deep sleep, 12 to 18 years old 8.5 to 10 hours of sleep and 1.7 to 2 hours of deep sleep, and those over 18 years old: 7.5 to 9 hours of sleep and 1.5 to 1.8 hours of deep sleep.

How Do Sleep Stages Change Throughout Life?

During a typical lifespan, the amount of sleep a person needs will change. Newborns sleep from 16 to 20 hours per day while kids between one and four will need 11 or 12 hours per day. This gradual decline continues into their adult life.
Not only does the body’s sleep requirements change, but its sleep patterns do as well. Newborns and infants typically spend about twice as much time in REM sleep as adults. The process that paralyzes the limbs, however, doesn’t develop until around six months, so many infants will wake due to movement while dreaming.
Infants and children will also experience much more of the restorative, non-dreaming deep sleep — stage 3. This begins to decline in early adulthood. Older adults will experience shorter periods of deep sleep and fewer of them. As one ages, sleep is lighter and more fragmented.
Understanding how sleep stages work can help one identify which patterns they’re experiencing and how they impact life overall. Of course, everyone is slightly different, but if one feels they’re experiencing sleep disturbances or aren’t getting good rest, their mattress might be to blame.
Everyone is set up for success with a mattress that’s tailored to their specific needs, and a comfortable, breathable sheet set to match!