Stages of Sleep
Don’t let your closed eyes and relaxed muscles fool you. You’re pretty busy while you’re sleeping. Read what happens during each sleep stage and why it matters.
Apr 22nd, 2021 •
Expert Insights from Dr. Anna Pickering a medical writer who received an honors baccalaureate of science in biochemistry and biophysics, minoring in chemistry, from Oregon State University and her doctorate in cell and molecular biology from the University of Hawaii at Manoa’s John A. Burns School of Medicine.
Going to sleep seems like a pretty straightforward process. You get tired, you power down, climb in bed, and fall asleep. As far as you’re concerned, nothing much happens while you’re sleeping. Maybe a dream or two, maybe a bathroom break, but the hours spent sleeping seem pretty uneventful until you wake up the next morning.
The occasional late night of socializing, TV binging, or cramming for a final exam, may not seem like such a big deal. You’ll be tired and cranky the next day, but you’ll catch up the next night, right?
Hold on, Night Owl. While you are sleeping, your body and brain are busy processing the previous day and getting ready for the day ahead. In fact, if you get your recommended seven to nine hours of sleep, your brain cycles through four distinct stages of sleep five or six times before you wake up.
Each stage of sleep has its own characteristics and function, although not all functions are known yet. Researchers have defined these sleep stages by identifying the brainwave activity that occurs at each stage. The different types and stages of sleep can be best identified using polysomnography, which simultaneously measures several body functions such as:
- Brain wave activity (electroencephalogram or EEG)
- Eye movement (electrooculogram or EOG)
- Muscle activity (electromyogram or EMG)
- Heart rhythm
- And more
A simplified summary of these results can be combined into a graph called a hypnogram, which gives a useful visual cross-section of sleep patterns and sleep architecture (learn more on sleep measurement). Although the process of falling into a deeper and deeper sleep is a continuum, with no sharp delineating markers, these sleep stages provide a convenient, if somewhat simplistic, means of describing the depth and type of sleep as the sleep period progresses.
There are two main broad types of sleep, each with its own distinct physiological, neurological and psychological features: rapid eye movement (REM) sleep and non-rapid eye movement (non-REM or NREM) sleep, the latter of which can in turn be divided into three or four separate stages. NREM sleep is sometimes referred to as “quiet sleep” and REM as “active sleep,” although these are not scientific terms.
Related: Sleep wake homeostasis
Electroencephalogram (EEG) traces of brain wave activity reveal that, in general terms, non-REM sleep is characterized by very slow but relatively high amplitude or high voltage oscillations, while REM sleep shows a much faster and lower amplitude trace, much more similar to normal waking activity (see diagram above).
Also, brain waves during non-REM sleep tend to be highly synchronized, and those during REM sleep much more unsynchronized. Electrooculogram (EOG) traces of eye movement indicate rapid eye movements during REM sleep, and little or no eye movement during non-REM sleep.
Electromyogram (EMG) traces of skeletal muscle activity show that, while the body is effectively completely paralyzed during REM sleep, the body does make some limited movements during non-REM sleep, including a major change in body position about once every twenty or thirty minutes on average. Based on these characteristics, early sleep researcher William C. Dement has described non-REM sleep as an idling brain in a moving body, and REM sleep as an active hallucinating brain in a paralyzed body.
The Stages of Sleep Defined
It has been known that sleep goes in cycles since as early as 1937. The distinction between REM and non-REM sleep was established in 1953. Since then, the various sleep stages have been defined and redefined until we have the breakdown of types and stages we know today. The pie chart below illustrates the duration of sleep stages for a typical adult – this breakdown varies by age.
The particular sleep architecture of an individual over the course of an average night – the overall sleep time, the structure and pattern of sleep stages and phases, the time spent in non-REM and REM sleep, the timing and organization of sleep cycles, etc. – is best illustrated by means of a hypnogram (see example for a typical adult below), a simplified graphical representation of the results from a more detailed polysomnogram over a complete sleep period.
The sleep cycle begins with this light sleep stage during which you can be easily woken. As you begin to relax, feel drowsy, and fall asleep, your brain begins to slow down and produce slower alpha and theta waves.
During this stage, you are not quite asleep, and you may experience vivid sensations called hypnagogic hallucinations. That’s that weird feeling of falling you sometimes have as you’re drifting off to sleep. Or you might also think you hear someone calling your name during a hypnagogic hallucination. And that sudden startle that jerks you awake for no apparent reason? That’s called a hypnic jerk. This stage of sleep is relatively brief lasting up to seven minutes.
This is also a light stage of sleep, but it becomes a little harder to wake you up. Your eye movements stop, your heart rate slows, and your body temperature decreases. Stage N2 sleep is characterized by sudden bursts of rapid brainwaves called sleep spindles, the exact function of which remains a mystery. Slower brainwaves resume after these brief bursts of brain activity. N2 lasts about 20 minutes for your first sleep cycle of the night and then increases in duration for each subsequent cycle. For most people, N2 comprises 40% to 60% of adult sleep time.
Also called Slow-Wave Sleep (SWS), this sleep stage marks the beginning of deep sleep during which the brain produces slower delta waves and you become less responsive to external stimuli and are much harder to awaken.
During the latter part of N3 (sometimes known as N4) your brain produces more delta waves, and you fall into a deeper stage of sleep from which it is much harder to be awakened. That’s because some important restorative processes are occurring. During N3, your body repairs muscles and tissues, stimulates growth and development, boosts immune function, and stores energy for the next day. Phew!
The first REM Sleep stage occurs about 70 to 90 minutes after you fall asleep and lasts about 10 minutes. As the night progresses, REM Sleep lasts longer until the final one that may extend up to an hour.
During the previous sleep stages, your brain powers down gradually until it falls into Slow-Wave Sleep or deep sleep. However, things change dramatically during Rapid Eye Movement or REM Sleep. Your brain suddenly becomes more active, producing brainwaves similar to those it produces when you are awake and toggling between beta and theta waves.
Related: REM rebound sleep
Most dreams occur during this stage, and they are more vivid than the few dreams that may occur earlier in the sleep cycle. Your eyes move rapidly under your eyelids perhaps in response to your dreams, your heart rate and blood pressure increase, and your breathing becomes fast, irregular, and shallow.
Oddly, your voluntary muscles are temporarily paralyzed during this sleep stage. Experts believe that this paralysis may result from a cool evolutionary trick to protect humans from acting out their dreams and hurting themselves. REM Sleep is believed to be important in long-term memory consolidation and mood regulation.
Each sleep stage in any particular sleep cycle fulfills a distinct physiological and neurological function that’s necessary for the health of the body and mind. If sleep is interrupted or if certain stages are missing for any reason, their physiological functions are not fully executed, and the person may feel tired or groggy even after an apparently sufficient sleep period, a phenomenon known as sleep inertia.
Miss any of the required number of sleep stages, and you could be looking at a really ugly day ahead. And that’s just one of the many reasons why getting your required hours of sleep is so important.
Sleep progresses in a series of four or five more or less regular sleep cycles of non-REM and REM sleep throughout the night, sometimes referred to as ultradian rhythms (“ultradian” meaning within a day).
The first sleep cycle is typically around 90 minutes in length, with the succeeding cycles averaging around 100-120 minutes, although some individuals may have longer or shorter average cycles, and they are usually shorter in children.
Each cycle follows the stages of non-REM sleep (stage 1 – stage 2 – stage 3) and then, after a period in deep stage 3 slow-wave sleep, back through the stages (stage 3 – stage 2 – stage 1). Then, instead of waking, the sleeper may enter a short period of REM sleep, before going back through the stages (stage 1 – stage 2 – stage 3) in a new cycle (see the hypnogram below). As the night progresses, the time spent in deep stage 3 sleep decreases and the time spent in REM sleep increases, so that there is a greater proportion of stage 3 sleep earlier in the night, and a greater proportion of REM sleep later in the night, particularly during the final two sleep cycles.
Take a look at this sleep cycle chart below to see a normal sleep cycle.
Generally speaking, the deeper the level of sleep, the slower, stronger and more synchronized the brain waves become. Also, the deeper the level of sleep in the cycle, the higher the arousal threshold, meaning it is quite difficult to wake someone in stage 3 sleep, but relatively easy in stage 1 or REM sleep.
There is also a general tendency towards decreased muscle tone as deeper and deeper sleep stages are achieved, although in this case REM sleep is anomalous in that muscle tone is at its lowest during that stage, despite its relatively high brain wave activity and low arousal threshold.
Ways to Encourage Sufficient Sleep Stages and Cycles
In recent years, special alarm clocks have become available which purport to monitor a person’s sleep stages and cycles and only wake them during periods of light sleep, when the deleterious effects of this sleep inertia are least acute.
Sleep trackers may also be used to give a better idea of sleep patterns. They’re not 100% accurate, but they do help the user to understand their cycles more effectively.
Napping is also a good way to recover from fatigue, but it’s best to avoid resting for an entire sleep cycle. A break of 15-20 minutes is proven to be more effective, and the body won’t suffer from sleep inertia after such a short period of time. As REM sleep only begins after 90 minutes, the arousal threshold will still be high after a quick nap.
The sum total of your night’s sleep is actually a complex choreography of alternating brainwaves and variously timed sleep stages. As you progress through the night, the amount of time you spend in each stage of sleep changes. You tend to experience more NREM Sleep in the first half of the night shifting to more REM Sleep in the second half. That’s why you often wake from a dream or a series of dreams in the morning.
“As you get older, you may experience changes in your sleep cycle,” explains Dr. Pickering.
The amount of time spent in each cycle and stage will vary from person to person, with age being a major factor. Just like babies and adults, different situations call for a variety of different cycles.
Babies spend about half of their total period of sleep in REM Sleep. By two years old, they cut that time in half and replace it with deep sleep (N3). By adolescence, 40% of deep sleep is replaced by N2 Sleep where it will remain as you age.
Not only does deep sleep decrease through childhood, but as people get older, the time they spend in REM Sleep will also decrease. “Babies spend almost half of their total sleep in REM sleep, but as an adult, you spend only about 20% of your sleep time in REM sleep,” says Dr. Pickering. By the time people are 70 years old, they spend most of their night in N2 Sleep, which may account, at least in part, for their frequent wakings through the night.
The cycle is also resilient to change, to the point where attempts at systemic alteration with drugs had little effect on the subject. Sleep cycles aren’t to be messed with, no matter how hard humans have tried.
Sleep deprivation and a poor sleeping pattern can also contribute to sleep paralysis. This phenomena is attributed to REM, and it occurs either while falling asleep or when waking; the body has entered REM atonia but either remains or becomes conscious.
“The complexity of the way our brain divides up our sleep into stages suggests there are complex mechanisms occurring while we sleep that researchers are still working to fully understand,” says Dr. Pickering.
Sleep has generally been a difficult subject to understand, but coming to terms with the cycles has allowed us to gain insight into what makes it so important. Research suggests that the brain is constantly interpreting data while the body is comatose.
The complexity and sophistication of the way our brain divides up our sleep make it clear that something very important is going on while we’re snoozing. And your body is pretty determined to get what it needs even if you try to deny it.
For example, after a period of partial or total sleep deprivation, your brain will prioritize SWS at the expense of other stages and repay your debt to this stage of sleep in proportion to your sleep loss. It’s important to rest every day, and to catch up on lost sleep cycles whenever possible.
Dr. Anna Pickering is a freelance medical writer based in Portland, OR working for The Med Writers. She received an honors baccalaureate of science in biochemistry and biophysics, minoring in chemistry, from Oregon State University and her doctorate in cell and molecular biology from the University of Hawaii at Manoa’s John A. Burns School of Medicine. Before transitioning to writing she completed a year of postdoctoral research at Oregon Health and Science University and worked briefly at the biotech startup Ayumetrix.
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