Article Author | Noah S. Siegel, MD | Massachusetts Eye and Ear Infirmary
What is sleep and the stages of sleep?
Sleep and the act of sleeping is a natural function common to all humans and animals. We are designed to function on a wake/sleep cycle, although with the advent of artificial light (thank you, Edison for the 1879 introduction of the light bulb), energy boosting drinks, foods, and medications, that cycle can be severely disrupted. It is clear that daily sleep is the preferred default setting for humans and that sleeping serves some very important functions that underpin health and wellbeing. However, there isn’t a complete agreement on what the functions of sleep actually are.
Sleep can simply be described as a state of reduced sensory and environmental responsiveness and physical inactivity. By comparison, wakefulness is characterized by, sensory perception, thinking, environmental responsiveness and physical activity.
There are various stages of sleep in mammals; the two most prominent are Rapid Eye Movement (REM) and non-REM (NREM) sleep.
In REM sleep, muscles are effectively paralyzed, what is called atonia, and dreaming occurs. Obviously, it’s a good design feature to be paralyzed while dreaming otherwise many of us would be sleepwalking and acting out or dreams with physical actions. This atonia is achieved through muscle inhibition by parts of the brain that regulate movement. Interestingly, there is a sleep disorder called REM Behavior Disorder which is characterized by individuals acting out their dreams.
In REM there is also an increase in breathing and heart rate variability. In addition, in REM the brain also uses a lot of energy, which is important because one theory of sleep is that it is about energy conservation, which might seem paradoxical. There is obviously a lot of mental activity in REM unlike in the other sleep phase, which has earned the REM phase of sleep “paradoxical sleep.” Core temperature is less well regulated in REM but sexual arousal is common and independent of dream content. In other words, physiology comes first and arousal is experienced, which may or may not be incorporated into a dream’s content.
NREM sleep is characterized as featuring general immobility, regular respiration and heart rate, and slow mental activity. It is divided into three parts:
- N1: falling asleep, just in that border between nodding off but still easily awoken
- N2: breathing and heart rate slow as you drift off to sleep
- N3: the slow wave delta phase that characterizes NREM. The hallmark feature of N3 sleep is slow, high amplitude delta waves on EEG. The first episode lasts 45-90 minutes but gets progressively shorter through the night. Children tend to have more N3 sleep than older individuals.
In addition to these separate and distinct sleep states, there is also a typical pattern of sleep in humans as we move in and out of these different phases. These two phases of sleep are so different that they have led to speculation that there is more than one function of sleep and those functions are represented by these quite different states.
Brain Wave Activity
- Delta: 1-4 cycles per second: Deep sleep. Typical NREM phase
- Theta: 5-8 cycles per second: Conscious, but low level of brain activation, e.g. meditation.
- Alpha: 9-13 cycles per second: Relaxed wakefulness
- Beta: 14-30 cycles per second: Active processing, stress.
Sleep occurs in cycles that typically last 90 minutes; the ultradian sleep cycle. Sleep proceeds from NREM phase to a REM phase, about five times a night. There is typically more NREM in the earlier part of sleep and more REM in the later part of the night or early morning. This is why people commonly awaken during their dreams. REM accounts for between 20% and 25% of total sleep time.
The way in which a person cycles through these phases, as well as the quality of the stages of sleep, determines the healthiness or otherwise of sleep. There are almost 80 distinct types of sleep disorders which reflect different dysfunctions in the stages and phases of sleep.
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