Many people believe that the brain "turns off" to rest during sleep . However, this is not necessarily the case: there is much more going on behind the eyelids than we think. The brain goes through several different cycles during sleep .
These cycles evolve according to different stages of sleep , starting with NREM (non-rapid eye movement) sleep and progressing to REM (rapid eye movement) sleep . A typical person begins a new sleep cycle every 90 to 120 minutes, which means they likely go through four or five cycles during a night's rest.
Here we will discuss the different stages of sleep , which are the most important, the amount of sleep needed and the evolution of sleep throughout life.
Stages 1-3: NREM sleep , lasts approximately 4 to 7 hours per night.
Stage R: REM sleep lasts 90 to 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 with very light sleep and progress to deeper sleep . At the end of a sleep cycle, the person moves from REM sleep to shallower sleep , returning to the lightest phase of sleep . The complete cycle then begins again.
The sleep cycle begins with stage 1 sleep . This is the time when the body begins to relax and sleep . This phase is often identified by the slow, rolling movements of a person's eyes and their ability to wake up easily.
At this point, brain wave activity begins to slow down to theta waves, while the body falls asleep. Some people may feel sudden jerks or muscle spasms during this phase or feel like they are falling. If the person is awake during this time, they will likely think they were not sleeping at all. Stage 1 usually lasts about 10 minutes.
The first phase where the person is completely asleep is stage 2, which tends to last longer than stage 1. During this period, the person is not as easy to wake up and their eyes roll, which shifts slowly, usually cease. The body temperature begins to drop and the heartbeat slows down.
Brain waves continue to slow down as the person drifts deeper into sleep . However, during this phase, we also see bursts of rapid activity, called sleep spindles . After a spindle, brain waves slow down again. Typically, stage 2 accounts for 40-60% of total sleep time.
When the body begins to fall asleep , it enters stage 3. This is a restorative stage that generally does not last as long as stage 2. It accounts for 5-15% of total sleep time for most adults. However, children and adolescents generally spend much more time in this stage.
During this deep sleep , brain waves slow down to delta waves, as the body prepares for REM sleep . During this time, the body strengthens its immune system, repairs and regenerates tissues, and builds bones and muscles. This is also when parasomnias such as sleepwalking, talking or night terrors occur.
During stage 3 sleep , it is much harder for the body to wake up – if a person wakes up, they will likely feel disoriented at first. When the body produces more delta brain waves at this stage, it moves closer to REM sleep.
Stage R (paradoxical sleep)
The fourth phase, called stage R or REM sleep , is the deepest of the four phases. It is at this stage that we generally have the most intense dreams. The first phase of REM sleep of the night is generally shorter and begins around 90 minutes after falling asleep, lasting around 10 minutes.
Throughout the night, periods of REM sleep lengthen as sleep cycles follow one another, ending between 70 and 100 minutes. The average adult experiences five or six REM sleep cycles during a sleep period.
During this period, the brain becomes more active. The person's eyes begin to move rapidly in different directions, their blood pressure and heart rate increase, and their breathing may become irregular. The muscles of the arms and legs are paralyzed, which prevents the person from achieving their dreams.
REM sleep helps the brain consolidate and process information from the previous day and store it in long-term memory. The older a person gets, the less time they spend in REM sleep.
Chart showing light sleep and deep sleep cycles. Over an 8-hour period, the body oscillates between light sleep and deep sleep approximately every hour.
What are the most important stages of sleep?
Not all stages of sleep are equal – some are more important than others. The work that occurs during stage 3 and REM sleep is different but equally important. These two stages are considered deep sleep , which is important for growth, hormonal regulation, and physical renewal.
During REM sleep , the brain processes memories and emotions, allowing humans to learn and think at a higher level.
If a person were to forgo deep sleep , they would likely feel sick and depressed, and even gain weight. Her cognitive processes would begin to slow down and she may have difficulty concentrating and socializing.
When does REM sleep occur?
REM sleep usually occurs around 90 minutes after falling asleep and the first session lasts around 10 minutes. As the person continues through their sleep cycle, the phases of REM sleep lengthen, with the final phase lasting about an hour.
How much deep sleep is needed?
Studies show that the average adult needs 1.6 to 2.25 hours of deep sleep per night. Since deep sleep is very important for brain health and function, it is imperative that each person spends 20-25% of their sleeping time in this state.
Children and babies need more time in deep sleep as their bodies are growing. Children up to the age of two need about 4.5 hours of deep sleep . Those between the ages of two and twelve need a little less – around four hours. Teenagers and young adults need about three hours of deep sleep.
Here is a list of sleep needs at different ages:
Newborn to 3 months: 12 to 18 hours of sleep , 2.4 to 3.6 hours of deep sleep ,
3 months to 1 year: 14 to 15 hours of sleep and 2.8 to 3.0 hours of deep sleep ,
1 to 3 years: 12 to 14 hours of sleep and 2.4 to 2.8 hours of deep sleep ,
3 to 5 years: 11 to 13 hours of sleep and 2.2 to 2.6 hours of deep sleep ,
5 to 12 years: 10 to 11 hours of sleep and 2 to 2.2 hours of deep sleep ,
12 to 18 years: 8.5 to 10 hours of sleep and 1.7 to 2 hours of deep sleep ,
Over 18 years old: 7.5 to 9 hours of sleep and 1.5 to 1.8 hours of deep sleep .
How do sleep phases evolve over the course of life?
Over the course of a normal life, the amount of sleep a person needs changes. Newborns sleep 16 to 20 hours a day, while children ages 1 to 4 need 11 to 12 hours a day. This gradual decline continues into adulthood.
Not only does the body's sleep needs change, but so do sleep patterns. Newborns and infants typically spend twice as much time in REM sleep as adults. The process that paralyzes the limbs, however, does not develop until about six months of age, so many infants wake from movement while dreaming.
Infants and children also spend much more time in deep, restorative, dreamless sleep , stage 3. This stage begins to decline in early adulthood. Older adults experience shorter and fewer periods of deep sleep . With age, sleep becomes lighter and more fragmented.
Understanding how sleep stages work can help identify patterns we experience and their impact on life in general. Of course, every person is slightly different, but if a person feels like they're having trouble sleeping or not getting enough rest, their mattress could be to blame.
Everyone is set up for success with a mattress tailored to their specific needs and a set of comfortable, breathable sheets to match!