The Sleep Stages: Your Brain’s REM and NREM Sleep Cycle

Long Story Short

• Your brain cycles through the sleep cycle 4 to 6 times each night. Each cycle is comprised of the four sleep stages: non-REM 1, non-REM 2, non-REM 3, and finally, REM. (1)
• Out of all sleep stages, we spend most of our time (45 percent) in non-REM 2. (1)  
• Non-REM 3, the third stage of the sleep cycle, is the deepest sleep stage and the one that helps you feel refreshed and restored in the morning. (2)
• The REM sleep stage is the one most commonly associated with vivid dreams, but it is not when you get your deepest, most restorative sleep. (3)
• REM sleep plays an important role in consolidating memories, emotion regulation, and brain development. (4)(5)(6)
• Age, sleep disorders, and lifestyle choices can affect how you experience sleep cycles.

When our heads hit the pillow each night, it’s lights out for the most part. And while sleep was once thought to be purely passive, research has shown it’s anything but. We may spend the next hours relatively unaware of what’s going on around us, but under the cover of night, our bodies are fantastically busy shoring up our immune system, building muscle, repairing tissue, and filing away memories — just to name a few. In order to do all of the above each night, we cycle through an assortment of sleep stages. And while the rapid eye movement stage (REM) might be the most well-known, the other superstar sleep cycle stages do some pretty heavy lifting to leave you rested and rejuvenated day after day.  

Note: The content on Sleepopolis is meant to be informative in nature, but it shouldn’t be taken as medical advice, and it shouldn’t take the place of medical advice and supervision from a trained professional. If you feel you may be suffering from any sleep disorder or medical condition, please see your healthcare provider immediately. 

What Is The Sleep Cycle?

Sleep is broken down into two phases: nonrapid eye movement (NREM) sleep and rapid eye movement (REM) sleep. These phases are then divided into four stages — N1, N2, N3, and REM — in which the sleeper moves from light to deep sleep, then back again. (1) When your body moves through all of these stages throughout the night repeatedly, those are called sleep cycles. These cycles usually last about 90 minutes, and most people go through four to six cycles each night. (1) Night after night, we go through the same sleep-wake cycle, which is regulated by our circadian rhythm. (1)

While sleep is still largely shrouded in mystery, we know that it plays a vital role in our overall health and well-being. While we lay snug in our beds each night, sleep helps us with: (1)

• Brain development/ maturation
• Memory consolidation and learning 
• Cognition 
• Improved immune functioning
• Healing, tissue repair, muscle growth, and bone building
• Clearing metabolic waste

How Long Is a Sleep Cycle?

A complete sleep cycle typically lasts anywhere from 90 to 110 minutes. With each cycle, sleepers progressively move through the N1, N2, N3, and REM sleep stages, with the new cycle beginning back in light sleep. A full night’s sleep usually includes four to six sleep cycles. (1)

The Four Stages of Sleep

Each night, our bodies cycle through 2 phases of sleep (NREM and REM) consisting of four sleep stages: non-REM 1, non-REM 2, non-REM 3, and REM.

Sleep Stage	Type of Sleep	Function
NREM 1	Non-REM	Light sleep. The transition between being awake and asleep. It’s still easy to wake up out of this stage.
NREM 2	Non-REM	Deeper sleep. Your body temperature drops, and your heart rate slows down.
NREM 3	Non-REM	Deep sleep. Your body is releasing growth hormone, repairing and regrowing tissue, and strengthening the immune system.
REM Sleep	REM	The stage most often linked to dreaming and is less restful than other sleep stages. Brain activity resembles that of a waking state.

Source: (1)

NREM 1 Sleep Stage 

N1 is the first and lightest stage of sleep. N1 is a dynamic state that represents the transition between a waking state and sleep. (1) It can last anywhere from one to five minutes and typically accounts for 5 percent of our total sleep time. (1)

During N1, the following physiological changes occur:

• Slow, rolling eye movement (7)
• Brain waves shift from the alpha waves of relaxation to the theta waves of sleep (1)
• Muscles begin to relax (1)

Sleepers may be roused quite easily from this stage of sleep, and small shifts back and forth between alpha and theta brain waves during N1 sleep may give the sleeper the sense that they are still awake. (1)

Alpha waves are a pattern of brain activity associated with relaxed wakefulness, whereas theta waves brain activity is associated with early stages of drowsiness or light sleep. (

Challenges with N1 sleep, such as extended sleep latency (the time spent waiting to fall asleep), may indicate the presence of a sleep disorder. Difficulty transitioning into N1 sleep or initiating sleep is also a common symptom of insomnia. (9)

NREM 2 Sleep Stage

N2 moves us into deeper sleep. Its duration changes as we progress through the night — the first N2 stage typically lasts around 25 minutes, and subsequent N2 stages grow longer as the hours tick by. (1) N2 sleep accounts for approximately 45 percent of our total sleep time, making it the sleep stage where we spend the most time. (1) Brain wave activity during N2 sleep includes sleep spindles and K-complexes (or long delta waves). While sleep spindles and K-complexes both play a central role in memory consolidation, K-complexes also play a role in maintaining sleep. (1)

While we sleep, we still experience environmental stimuli — some are harmless, while others can be dangerous enough to warrant us waking up and taking action. K complexes (a pattern of brain wave activity that typically occurs during NREM stage 2 sleep) are thought to protect sleep by preventing waking to normal stimuli that do not present a danger. (10)

During N2, the following physiological changes occur: (1)

• Decrease in heart rate and body temperature
• Blood pressure drops (11)
• Breathing is less erratic (11)

NREM 3 Sleep Stage

N3 is the deepest stage of sleep, and out of all the sleep stages, it has the most aliases. N3 is also known as slow-wave sleep (SWS), deep sleep, and delta sleep (due to the brain wave activity that occurs during this sleep stage). N3 accounts for 25 percent of our total sleep time. It’s usually the hardest to wake up from and, therefore, the one most commonly associated with sleep inertia. (1)

While N3 plays a heavy hand in how restored and refreshed we feel upon waking, it’s the stage where biological processes like growth hormone release, bone and muscle building, and immune strengthening occur. (1) As they age, older adults spend less time in N3 sleep and more time in N2. (1)

REM Sleep Stage

The rapid eye movement stage, or REM sleep as it’s more commonly known, is the fourth and final sleep stage in a sleep cycle. Brainwave activity shows beta waves, which closely resembles brainwave activity in a waking state. (1) REM sleep is:

• The stage where most dreaming occurs. 
• The sleep stage from which dreamers are more likely to remember their dreams. 
• Typically considered less restful than other sleep stages. (3) 

Additionally, the amount of time spent in REM sleep tends to decrease with age. While infants spend as much as 50 percent of sleep time in the REM stage, adults may spend closer to 25 percent. (12)

Most sleepers will hit their first REM stage somewhere around 60 to 90 minutes into their sleep cycle. (1) Initial REM stages are short and continue to lengthen as the night wears on. As such, your first REM stage of the night could be as short as 10 minutes, whereas your last REM stage could last as long as one hour. (1)

During REM sleep, the following physiological changes occur: (1)

• Erratic breathing and heart rate
• Muscle paralysis (muscle atonia) 
• Rapid eye movements

Deep Sleep vs REM Sleep: What’s the Difference?

REM sleep and deep sleep are two distinct sleep stages, each with their own unique benefits

Deep sleep (aka N3, slow wave sleep, or delta sleep) is the third stage of the sleep cycle and a part of non-REM sleep. It’s the deepest stage of sleep (hence its name) and the most restful and restorative. Deep sleep is also where some critical biological housekeeping occurs — including growth hormone release, muscle and tissue growth and repair, and immune system fortification. (1)

REM sleep, though different, is equally important. This is the stage where most of our dreaming occurs, and while it’s often confused with deep sleep, it’s typically the least restorative than the other sleep stages. (1) Other critical functions of REM sleep include memory consolidation, emotional balance, and brain development. (13, 14)

What Affects Sleep Cycles?

Night after night, our sleep cycles are essentially on autopilot. We turn off the lights, our heads hit the pillow, and the cycle begins. However, Dan Gartenberg, a sleep scientist and sleep health advisor for CPAP.com, says some things, like age, sleep disorders, and lifestyle choices, may meddle with our sleep cycles. 

Age

“Age is a notable factor that can affect your quality or duration of sleep within the sleep cycle; as such, sleep patterns generally change as you age,” Gartenberg tells Sleepopolis. For example, while infants spend about 50 percent of their total sleep time in REM, that levels off as they mature, eventually falling more in line with adult REM sleep requirements. (11)

Moreover, Gartenberg tells us that wonky sleep cycles are par for the course of aging. While adult sleep requirements don’t change with age (healthy adults 18 and over should get about seven to nine hours of sleep each night), their actual sleep experience often does. (15) “Older individuals tend to wake up more in the middle of the night or earlier in the morning,” says Gartenberg, noting that it’s often tied to “underlying conditions and medications.” Pair that with the host of hormonal and physiological changes that accompany aging, and it becomes clear why older individuals tend to have altered sleep cycles. 

We’ll add here that with age, adults spend less time in N3 or deep sleep. (1)

Sleep Disorders

“Sleep disorders can also negatively impact sleep cycles,” says Gartenberg. While some sleep disorders like insomnia can put up roadblocks to even getting the sleep cycle started, others like obstructive sleep apnea and restless leg syndrome can cause frequent wakings that impair our progression through the cycle or curtail our time in any given sleep stage. (16)

Health Conditions

Physical and mental health conditions can also affect your sleep cycles each night. For example, studies have shown that obesity can increase awake time and reduce sleep efficiency. (17) Additionally, some mental health conditions like depression and anxiety can also stymie sleep, with research showing that impairments are evident across all sleep stages. (18) (19)

Lifestyle

“Lifestyle choices can affect sleep cycles as well,” says Gartenberg. “Someone who exercises consistently will likely find that they have better sleep quality. Meanwhile, those who use their devices before bed are more likely to have trouble falling asleep at night, as the blue light emitted from the devices can interfere with melatonin production, ultimately making it harder to fall asleep.” Other lifestyle factors that can have a negative impact on your sleep cycles include the consumption of caffeine, nicotine, and alcohol. (20) (21)

Supporting a Healthy Sleep Cycle

Far from being a passive process, the functions that occur during sleep are essential to our health and well-being. To protect our sleep health and encourage restful and restorative sleep, meeting our daily sleep requirements is crucial, as is practicing healthy sleep hygiene.

The basics of sleep hygiene include: (21)

• Maintaining regular sleep and wake times
• Limiting or eliminating noise, excessive light, and extreme temperatures in either direction in the sleeping environment
• Avoiding caffeine and alcohol before bedtime
• Reducing the use of electronics in the hours leading up to sleep, as many of these emit a blue light which decreases melatonin production and promotes alertness
• Napping strategically 
• Reserving the bed only for sleep and sex
• Exercising regularly 

FAQs

The Last Word From Sleepopolis 

Sleep is a dynamic state that evolves as our bodies change over a lifetime. From growth and tissue repair to memory consolidation, the processes that occur during sleep are essential to the healthy functioning of the brain and body. 

Unfortunately, sufficient (and healthy) sleep is sacrificed to busy work schedules and family routines all too often. However, a deeper understanding of sleep’s importance may be the key to changing how we prioritize rest and activity. Ultimately, healthier habits and sleep-promoting routines may help the body capitalize on all four stages of sleep to benefit our long-term health.

Sources

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