The Human Sleep Cycle
Have you ever seen a photograph of sleeping children and thought how peaceful they look? That restful appearance is quite deceptive, because a sleeping person experiences constant brain activity, often coupled with movements of the muscles. Sleep is not passive but active, yet we need it to feel rested.
As your body descends through the successive stages of sleep, your brain waves and muscle movements change in predictable ways, allowing researchers to identify each stage of sleep. Some of these physical shifts are measured using three machines, which produce the electroencephalogram (EEG) for brain waves, the electromyogram (EMG) machine for skeletal muscles, and the electrooculogram (EOG) for eye movements.
The unified recording technique, performed in sleep laboratories, is called polysomnography. Polysomnography records the following physical processes: brain waves, eye movement, muscle tone, breathing, blood pressure, and heartbeat during sleep. It can also include EKG (electrocardiogram) monitoring specifically.
One interesting aspect of sleep is its restorative quality: your nerves, muscles, bones, and immune system all recover from stress and even undergo growth (in a process called anabolism, or the metabolic activities that build new cells) while you sleep.
Five Stages of Sleep
There are five well-established phases of sleep, and they tend to follow each other in the same order, which we call the sleep cycle. There are also known average times for each stage, making it very easy to identify sleep disorders in a laboratory setting (by looking for a stage that's too long or too short, or one that occurs in the wrong place in the cycle).
During a complete sleep cycle, which usually lasts between 90 and 110 minutes, your body passes through stages 1 and 2 (which occupies 50% of sleep time), 3, 4, and REM or "rapid eye movement" sleep (taking 20% of sleep time). Stages 1, 3, and 4 together account for the other 30% of your sleeping hours. An eight-hour period of sleep usually contains between four and five cycles.
Stages 1 and 2 (AASM Stages N1 and N2)
Stage 1 consists of very light sleep, from which you can easily be awakened. Your eyes move slowly, your muscle activity decreases, and your brain waves slow down. In this borderland between sleeping and waking, you'll sometimes encounter sudden muscle spasms called hypnic myoclonia, which resemble the startle reflex and give you the feeling of falling.
In this stage, your brain gears down from waking alpha waves (whose frequency ranges from 8 to 13 hertz) to theta waves (4–7 Hz). The onset of theta waves characterizes this phase, and if you see them graphed you'll notice they are both slower and greater in amplitude (up-and-down movement) than the waves generated in a waking state of relaxation, so they're visibly transitional in form.
In stage 2, your eyes stop moving, the EMG reveals that electrical activity of the skeletal muscles has slowed, and your brain waves (which are still theta waves) intermittently exhibit two new features, called sleep spindles and K-complexes.
Sleep spindles (also called sigma waves, 11–16 Hz) are clusters of small, rapid waves, usually preceding some muscle movement. They may help the brain combine new learning with learning already established. Schizophrenic patients display abnormal spindle patterns.
K-complexes look like single large waves (the voltage of which usually exceeds 100 millivolts), with a high peak and a low trough. They are thought to be the brain's way of signaling that it's safe to overlook an external stimulus, and also thought to help memory develop. People who are afflicted with restless legs while asleep show greater numbers of K-complexes.
Stages 3 and 4 (AASM Stage N3)
These stages are collectively referred to as deep sleep or slow wave sleep. A sleeper in this part of the cycle is very hard to awaken, and there's no measurable movement of eyes or muscles. Stage 3 presents very slow delta waves (less than 50% of brain activity) mingled with smaller, faster waves. Delta waves are the slowest of all brain waves, and they have the largest amplitude.
In Stage 4, the brain emits almost all delta waves, which look very different from waking alpha and beta waves. The brain has to change its wave activity over a period of time to reach the delta wave stage, normally making a gradual shift from alpha or beta to delta.
It should be noted that the American Academy of Sleep Medicine (AASM) changed its sleep stage classification in 2007, combining Stages 3 and 4 into a new stage numbered N3. Stage N3 begins when the brain achieves at least 20% delta waves. People who walk or talk while sleeping, experience night terrors, or wet the bed, typically do so during Stage N3.
Rapid Eye Movement (REM) Sleep
REM sleep, in which most (but not all) dreaming occurs, brings notable physical changes. Your breathing becomes more rapid, irregular, and shallow, and your heart rate rises. Your eyes dart quickly to and fro. You experience atonia caused by the release of an amino acid called glycine in the brain.
Atonia is the loss of normal muscular tension in leg and arm muscles, resulting in temporary paralysis. Many researchers believe this is an evolutionary adaptation to prevent the sleeper from physically replicating movements made during vivid dreams. Some ability to regulate body temperature is lost, and male subjects develop penile erections (which means that effect can distinguish physical erectile dysfunction from the psychological variety).
Most interesting of all, the brain waves seen during REM sleep look like waking brainwaves interspersed with very irregular waves, just as REM sleep has some qualities of wakefulness combined with unusual effects like dreams and unmoving limb muscles. Your waking EEG and your REM EEG look quite similar, including small, fast waves as opposed to the large, long waves of non-REM sleep.
Therefore, the EMG that measures muscular tension (there's no muscle activity in REM sleep) is needed to pinpoint that important difference. The portion of the brain stem responsible for REM sleep is called the pontine tegmentum, containing the neurons known as REM sleep-on cells, which become active during the REM stage.
REM sleep begins between 70 and 90 minutes into your first sleep cycle. Your body has a particular need for REM, and if it's suppressed (for example, by substances like alcohol or antidepressants), your next sleep cycle contains extra REM time until you make up the deficit.
Deprivation of REM sleep causes anxiety and irritability. There are various theories about what REM does. One theory holds that two types of memory, procedural and spatial, are consolidated in REM. Memory consolidation refers to the physical stabilization of a memory trace in the brain after it has been acquired.
Other theories suggest REM helps the central nervous system develop, or that the brain needs to take a rest from neurotransmitters called monoamines, which are not released at all during REM sleep. That rest period gives monoamine receptors time to recover so normal brain function can resume.
Changes in Sleep Cycles
Sleep cycles change during the night, and during a person's lifetime. In your first sleep cycle during the night, stages 3 and 4 are long, but your REM phase is short. Gradually 3 and 4 shorten while REM lengthens. By the time morning comes, most of your sleep consists of stages 1 and 2, plus REM sleep. If you sleep longer than a few minutes, you might not recall the few minutes just before you fell asleep, and that's normal.
Your age determines the amount of time you spend in each stage of sleep. Babies sleep a lot, and their sleep includes more REM. As an example, a newborn might sleep 16 hours each day, and spend half that time in REM. But people between 50 and 85 might sleep only six hours a day, spending a mere 15% of that time in REM.
In animal studies, more physical exercise results in more non-REM sleep. That helps us understand why a person deprived of stage 4 sleep feels physically tired.
All the research conducted on sleep has resulted in some useful practical tips on optimizing your sleep experience. For example, there are now alarm clocks that can tell which sleep stage your body is in and awaken you during light sleep, which will help you feel refreshed rather than groggy first thing in the morning.