Stages of Sleep:
A Guide to NREM, Deep, & REM Sleep Cycles
Sleep is something most of us just take for granted. And, unless we’re not getting enough sleep, it’s something we don’t really think too much about.
But what actually happens when we sleep?
It might seem like our body just kind of shuts down and goes into rest and recovery mode, but the human body, and especially the brain, is wonderfully complex and more active than you’re probably aware while sleeping.
This guide is going to give you a good understanding of:
- the different stages of sleep and sleep cycles
- what happens inside your brain while you’re asleep, or dreaming
- the body’s biological sleep mechanisms and circadian rhythm
By the time you’ve read this guide, you’ll have a deeper appreciation for sleep and why it’s so important to ensure you’re getting both enough sleep and good quality sleep.
A Brief Scientific History
The study of sleep and what we know about it has a relatively short history, with modern sleep research only really beginning in the 1920s.
Prior to this era, scientists believed that the brain essentially “shut down” during sleep; that brain function diminished along with sensory inputs while we sleep.
The Electroencephalogram (EEG)
The invention of the electroencephalogram (EEG) in 1929 allowed scientists to “take a picture” of the brain’s electrical activity and proved that far from shutting down, the brain is at times highly active during sleep.
Two Main Types of Sleep
Further studies that measured eye movement and muscle activity along with brain activity resulted in the discovery of two types of sleep — Non-rapid eye movement (NREM) sleep and rapid eye movement (REM) sleep.
NREM sleep can be broken down into three distinct stages (it was formerly four, but stages 3 & 4 have since officially been combined into one) and are characterized by the slowing down and synchronization of brainwaves.
During stage 3, the deepest sleep, brain waves are very large and slow (high amplitude, low frequency).
During REM, or active sleep, brain waves are small and fast (low amplitude, high frequency). This stage of sleep is also accompanied by the eye movement that gives this sleep stage its name. This is also the stage associated the most with dreaming, which we will get to a bit further on.
The Stages of Sleep
Let’s now take a closer look at each of the individual sleep stages and what happens during each one:
During the pre-sleep stage, brainwaves begin to slow and change from their wakeful pattern. During this very early sleep stage, the brain produces alpha waves.
These slower waves are associated with being in a restful state, and the brain produces them during meditation, deep prayer, and other similar activities.
Breathing, heart rate, and eye movements slow down. Muscles will begin to relax, but people in this sleep stage may experience what’s known as myoclonic jerking — those inexplicable sudden jerks we’ve all experienced when drifting off to sleep.
Hypnagogic hallucinations may accompany this jerking — this is when you feel like you’re falling or hear a distinct noise like someone saying your name, or a doorbell ringing, for instance.
1. NREM 1
Level 1 of non-REM sleep is a very short period (only lasting 5 to 10 minutes) that essentially just acts as a changeover from a state of wakefulness to sleep.
NREM 1 is the “catnap” stage — if you take a brief nap during the day, this is the stage you’ll be in for most if not all of your nap, and it is the stage where you are most easily able to be disturbed or roused.
This stage is such a light transitional stage that if you are woken while in this stage, you may claim you were not even asleep — “I was just resting my eyes!”
Brain activity slows even further in NREM 1, from producing alpha waves to theta waves (very slow, high-amplitude low-frequency waves).
2. NREM 2
This next sleep stage is still light and usually lasts around 20 minutes. During NREM 2, brain wave activity continues to slow, but it continues to produce short bursts of rapid, rhythmic activity known as sleep spindles.
The function of these sleep spindles remains unclear. Still, the prevailing belief is that they are to do with memory consolidation and moderation of response to sensory stimulation during sleep.
During NREM 2, heartbeat and breathing slow while muscles relax even further, body temperature drops, and eye movement completely stops.
We usually spend more time (about half our sleep time) in NREM 2 sleep than other stages over repeated sleep cycles.
3. NREM 3 (Deep Sleep)
NREM 3 is the “deep sleep” stage. It will last anywhere from 45 to 90 minutes during the first sleep cycle, and each stage gets shorter during subsequent cycles.
During NREM 3 sleep, the brain produces a very slow Delta wave activity. This is why NREM 3 is also known as Slow-Wave Sleep (SWS).
It is during this stage of deep sleep that heartbeat and breathing drop to their lowest levels, and muscles are at complete relaxation. The body is far less responsive to outside stimuli — it is hardest to rouse someone in this sleep stage.
Even though the muscles are generally completely relaxed, sleepwalking is most common during NREM 3 sleep.
This deep sleep stage is where rejuvenation occurs and is what allows you to wake up feeling refreshed.
During NREM 3, sleep muscle and tissue repair occur, growth and development are stimulated, immune function is boosted, and energy for the following day is built up.
NREM 3 sleep decreases as we get older. It is a myth that our need for sleep decreases as we get older, but we get less deep sleep — some older adults may have no measured NREM 3 sleep, but this is not abnormal.
We usually finally reach REM sleep about 90 minutes after falling asleep. On average, we’ll go through 3 to 6 REM stages each night, and each REM stage gets longer in later cycles; the last REM stage can last an hour.
REM sleep makes up about 20% of total sleep time in adults, and 50% for babies. REM sleep is characterized by the rapid flickering movement of the eyes that gives it its name.
Interestingly, studies suggest that these eye movements are connected with visual processing while dreaming, with eye movements corresponding with seeing a new mental image, so our eyes moving are likely us “seeing” things as we dream.
During REM sleep, brain activity increases, with wave activity closer to waking levels than in the other stages of sleep.
Heartbeat and blood pressure also return close to waking levels, and breathing becomes faster and more irregular, as opposed to the slow, rhythmic breathing of deep sleep.
As mentioned in reference to eye movement, REM sleep is the stage of dreaming. While dreams can and do occur during other sleep stages, most dreaming and the vivid dreams that we remember happen during REM sleep.
While our eyes are able to move and do so in correspondence with what we see during our dreams, our voluntary muscles become paralyzed. This is thought to be a protective measure — so that we can’t “act out” our dreams.
REM sleep is sometimes also known as Paradoxical Sleep since brain activity and other body systems become more active during this sleep stage, but muscles become more relaxed and immobilized.
It is also this muscle paralyzation that makes REM sleep the riskiest stage for sufferers of sleep apnea. It worsens during REM sleep due to the relaxation and paralysis of muscles worsening airway obstruction.
Other physical characteristics of REM sleep are that the body loses some of its ability to regulate temperature, and blood flow to the genitals increases, which is why involuntary night-time and morning erections in men are common.
The REM stage is also where the brain processes and consolidates information. During NREM sleep, it is thought that information is “passively” encoded to long-term memory while during REM sleep, it processes “procedural and emotional memory.”
The Sleep Cycle
We don’t just go through each stage of sleep once per night — these stages repeat in cycles throughout the night. The typical person will go through four or five cycles during each sleep, beginning a new cycle every 90-120 minutes.
People will typically spend more time in NREM sleep during earlier cycles, while later cycles will be more heavily weighted towards REM sleep.
During the final sleep cycle, the body may even skip the NREM deep sleep stage entirely and go into an extended REM sleep stage instead.
Overall, you will spend more time in NREM sleep during the night than in REM sleep.
A study has found that all people dream whether they remember their dreams or not.
Many people claim to not dream at all; the reality is they do — they just do not recall them upon waking. We may dream 4 to 6 times and spend up to 2 hours dreaming per night.
Dreams can be experienced in all sleep stages, but the ones during increased brain activity of REM sleep are most vivid and remembered.
It is not clear exactly why we dream or what purpose dreaming serves, but theories suggest some possible reasons:
- Confronting drama in your life and making connections regarding emotions and feelings that your conscious brain can’t or won’t.
- The amygdala, which deals with our survival instinct and the fight-or-flight response, is active during dreaming, so some dreams may be a form of “survival training.”
- Dreams may unlock our creativeness. Often, dreams are credited for instances of great clarity, discovery, ideas, and solutions to problems.
- Dreams may help us process emotions and feelings, and sort, prioritize, and either embed or discard memories, helping us store information without interference from other stimuli.
The Brain and Sleep
So much of what we know about what happens during sleep comes from studying the brain. So much varying activity occurs during sleep, as evidenced by the vast array of brainwaves that can be measured through the different sleep cycle stages.
The hypothalamus is small and peanut-shaped, located deep inside the brain. It essentially acts as the “control center” that switches our body between wakefulness and sleep.
Arousal signals in the form of neurotransmitters are sent to the brain’s largest area, the cerebral cortex. These arousal signals, when active, keep us awake.
One part of the hypothalamus called the ventrolateral preoptic nucleus (VLPO) contains arousal-inhibiting neurons. The signals from these neurons shut down the arousal centers of the brain, promoting sleep.
Also within the hypothalamus is the suprachiasmatic nucleus (SCN); clusters of cells that react to light exposure through the eyes.
The body’s circadian rhythms, which dictate when we sleep, are tied to the light-dark cycle, so people who have suffered damage to the SCN may suffer from erratic sleep. Most blind people can still sense light, so do not experience disruption to their circadian rhythms.
Located at the base of the brain, the brainstem communicates with the hypothalamus to control the transitions between wakefulness and sleep.
Neurons within the brainstem utilize excitatory and inhibitory amino acids that promote or inhibit wakefulness.
It is the inhibitory neurotransmitter gamma-aminobutyric acid (GABA) that blocks arousal signals in the brainstem and hypothalamus, decreasing nervous system activity.
It is the brain stem that is responsible for the signals that relax and paralyze our muscles during REM sleep.
Thalamus & Cerebral Cortex
The thalamus is the part of the brain, located near the center, that relays information received from our senses to the part of the brain that interprets and processes information from short to long-term memory — the cerebral cortex.
The thalamus becomes relatively inactive during deep sleep, which is why we don’t react easily to outside activity. Still, it is active during REM sleep, which is why the cerebral cortex is able to receive images and other stimuli from dreams.
The pineal gland is a small endocrine gland located between the brain’s two hemispheres in a fluid-filled space behind the third cerebral ventricle. It is about the size of a pea and gets its name from its pinecone shape.
The pineal gland contains pinealocytes; cells that produce melatonin, a sleep hormone produced according to the amount of light one is exposed to — it receives signals from the suprachiasmatic nucleus (SCN) in the hypothalamus and releases melatonin accordingly.
As its name suggests, the basal forebrain is near the bottom-front of the brain. It helps control sleep and wakefulness by producing adenosine (a chemical byproduct of cellular energy consumption), which supports the sleep drive. Caffeine can inhibit sleep due to its ability to block adenosine.
Adenosine promotes slow-wave sleep through inhibiting the release of acetylcholine, which induces both wakefulness and REM sleep.
The amygdalae are two almond-shaped clusters of nuclei located deep within the temporal lobes of the brain. They play a primary role in processing memory, emotional responses, and decision making.
The amygdala becomes very active during REM sleep, as this is when specific memory processing happens.
Due to the amygdala’s role in emotional response, it is believed sleep deprivation can cause and exacerbate anger, depression, anxiety, impulsiveness, and other forms of emotional instability.
Internal Biological Sleep Mechanisms
Circadian rhythms and homeostasis are two interconnected mechanisms that your body utilizes to regulate sleep and wakefulness.
An easy way to distinguish the two is to look at them like this: homeostasis tracks your body’s need for sleep, and your body’s circadian rhythm is responsible for the timing of sleep.
Your body’s circadian rhythm is what regulates internal changes within your brain and body over the course of a day, including sleep and wakefulness.
It is believed that organisms with circadian clocks gain an evolutionary advantage through extrinsic adaptive value (synchronizing behavior and physiological processes to cyclic environmental factors) and intrinsic adaptive value (coordinating internal metabolic processes).
Circadian is derived from the Latin word circa (“around” or “approximately”) and diem (“day”).
These rhythms are endogenous, or “built-in” to our bodies. Still, they can also be influenced by outside stimuli, meaning that they are adjusted to the external environment through cues such as light and temperature.
The body’s biological clock controls the circadian rhythm. This biological clock is a part of the brain we’ve already mentioned — the suprachiasmatic nucleus (SCN). The SCN receives light signals from the eye.
Signals then travel from the hypothalamus to the pineal gland, which regulates the release of the hormones that make us sleepy or awake.
The change in melatonin levels during the sleep/wake cycle reflect circadian rhythms. Seasonal changes can affect our body’s circadian rhythm, impacting our sleep and level of tiredness — we may feel tired more often during the peak of winter when there’s less natural light, and the sun goes down earlier.
Our circadian rhythm can change as we age, too. Older people are prone to desynchronized rhythms where they may go to bed earlier, get up earlier, and develop irregular, interrupted sleep patterns that may include taking naps during the day.
These changes may be related to neurological changes — the brain produces less growth hormone and melatonin as we age, which could account for shifting sleep patterns.
Changes in daily activities, including lack of structure post-retirement, may also be a factor in changing circadian rhythms and sleep patterns.
Circadian Rhythm Disruptors
There are external factors that are prevalent in modern life that can disrupt our natural circadian rhythm and sleep cycle:
- Irregular and inconsistent mealtimes
- Exposure to light within an hour of bedtime (blue light from device screens is especially bad)
- Light sources in the bedroom (lights from electronics, alarm clocks, streetlights, and other external lights)
- Not being exposed to bright light upon waking in the morning helps kickstart the cycle and other processes.
It is sleep-wake homeostasis that tells us it is time for sleep — it’s what creates the sleep drive in humans.
Quite simply, it works like this: the longer we are awake, the stronger the desire or need for sleep becomes, and the longer we’ve been asleep, the more the need or pressure for sleep dissipates, meaning we’re more likely to wake up.
But it doesn’t quite work like that — if it were sleep-wake homeostasis on its own that dictated sleep, we would simply get more and more tired the longer we’re awake, and we’d likely nap in cycles during the day as the drive for sleep would build until we succumbed, then dissipate as we napped.
And, while as a general rule, sleep pressure builds the longer we are awake, it is our circadian rhythm that actually dictates how sleepy or awake we feel throughout the day — we have peaks and troughs of wakefulness and tiredness throughout the day.
It is the circadian rhythm that allows us to overcome the drive for sleep and stay awake continuously for around 16 hours out of every day.
The Importance of Good Sleep
The sleep needs of each person are going to vary. As a general rule, we should all wake up feeling refreshed and not tired upon waking or needing naps throughout the day. Our sleep needs change as we age, and our sleep cycles differ at different stages of life.
A study by the National Sleep Foundation has resulted in the following sleep recommendation guidelines:
|Life Stage||Required Amount of Sleep (Hours)|
|Newborn (0-3 months)||14-17|
|Infant (4-11 months)||12-15|
|Toddler (1-2 years)||11-14|
|Preschool (3-5 years)||10-13|
|School-age (6-13 years)||9-11|
|Teenage (14-17 years)||8-10|
|Young Adult (18-25 years)||7-9|
|Adult (26-64 years)||7-9|
|Older Adult (≥65 years)||7-8|
As the amount of sleep required changes, so does the amount of time spent in each sleep stage and the timing of our sleep cycle.
Sleep cycles last around 50 minutes for children and 90 minutes for adults. Infants and children spend more time in deep NREM 3 sleep than adults, and this requirement for deep, slow-wave sleep diminishes from early adulthood onward.
Deep sleep in elderly adults comes in small, short bursts, if at all, and sleep overall is lighter, shorter, and more frequently disturbed as we get further into old age.
Unsurprisingly, older adults tend to suffer from chronic sleep deprivation due to not being able to maintain consistent sleep.
Side effects from health conditions that develop as we age also contribute to disturbance and lack of sleep. Treating these underlying conditions can greatly improve the amount and quality of sleep in older adults.
The Consequences of Insufficient Sleep
Aside from simple tiredness, there are many potential consequences of lack of sufficient sleep, both in the short and long term. Long-term sleep deprivation can actually contribute to serious health issues.
Short Term Consequences
- Ability to learn
- Ability to retain information
- Risk of accident and injury
Long Term Consequences
- Greater risk of disease and poor health
- Alcohol & drug dependence
- Hypertension and cardiovascular disease
- Lowered immunity
- Early mortality
Sleep is an inescapable fact of life, and a lot of what we know about it from a scientific perspective has only come about relatively recently, with new discoveries and insights still being made today.
What is fascinating is the evolutionary, hardwired nature of sleep — we all need it, we can’t escape it, and it’s dictated by multiple processes and reactions that happen in our brain and the rest of our body.
By now, you should have a better understanding of the bodily functions that affect and are affected by sleep; the way the sleep cycle works, and the different sleep stages that make up the sleep cycle, as well as their unique traits and purposes.
You should also have a good understanding of the importance of sleep and how our needs change as we age, so be sure you are getting the right amount of good quality sleep for your age and lifestyle — it’s one of the easiest things you can do for your overall health.