Q: Are there different states of sleep?
A: There are two states of normal human sleep: nonrapid eye movement sleep, or NREM sleep, and rapid eye movement sleep, or REM sleep. NREM and REM sleep usually alternate in cyclically with each other. Under normal circumstances, you enter sleep through NREM sleep and this cycles with REM sleep at about 90–110-minute intervals. Young adults usually experience 4 to 5 REM periods throughout the time they are asleep.
Q: What exactly is NREM sleep?
A: NREM sleep comprises light sleep and also slow wave sleep. By convention, NREM sleep is subdivided into 4 stages. Stages 1 and 2 are considered to be light sleep and stages 3 and 4 are called slow wave sleep. The 4 stages of NREM sleep parallel the depth of sleep. You are therefore most likely to waken easily in stage 1 and be very difficult to waken in stage 4 sleep. The best way to think of NREM sleep is as “a relatively inactive yet actively regulating brain in a movable body.”
Q: What exactly is REM sleep?
A: During REM sleep we have significant brain activity along with muscle atonia, which means that our muscles are effectively paralyzed, except for the muscles that move the eyes, the diaphragm (for breathing), and the heart. During REM sleep we experience episodic bursts of rapid eye movements. This is the period when we experience dreams. If woken from this stage of sleep, you are likely to have vivid dream recall about 80 percent of the time. The best way to think of REM sleep is as “a highly activated brain in a paralyzed body.”
Q: Is it normal to enter sleep through REM sleep?
A: Apart from circumstances such as “catch up” sleep after sleep deprivation, it is abnormal for an adult to enter sleep through REM sleep. But it is normal for infants up to the age of 3 months or so to have sleep-onset REM (also called active sleep). Between 3 and 6 months, NREM sleep (or inactive sleep) in infants becomes clearly demarcated.
Q: What are the different stages of sleep in adults?
A: Sleep occurs in cycles. The first cycle begins with stage 1 NREM sleep, when it is easy to wake someone. Stage 2 NREM sleep has a higher arousal threshold. This stage lasts about 10–25 minutes. Stages 3 and 4 of NREM sleep are characterized by slow wave sleep, which takes up about 20–40 minutes in the first sleep cycle. Body movement and lightening of sleep occurs within about 5–10 minutes of stage 2 prior to going into the first episode of REM sleep of the night. This first period of REM sleep usually lasts for 1–5 minutes.
Q: How long do the sleep cycles last?
A: NREM and REM stages of sleep alternate throughout the night in a circadian rhythm. In adults, the average duration of the first sleep cycle of the night is 70–100 minutes and later cycles last about 90–120 minutes. Slow wave sleep is dominant in the first third of the night and REM sleep is dominant in the last third. This is why we generally wake up from a dream or just after a dream in the morning.
Q: What determines how much sleep I get?
A: The amount of NREM sleep depends on the initiation of sleep, how long you have been awake, and the duration of sleep. The amount of REM sleep is linked to the body clock and to circadian changes in body temperature. The length of NREM and REM sleep also change with age.
Q: What happens during slow wave sleep?
A: During slow wave sleep, there is an increased secretion of growth hormone with about 70 percent of this hormone being produced during this stage. Slow wave sleep occurs in greater amounts during phases of growth (childhood into early adulthood), pregnancy, and breast-feeding (because of increased levels of the hormone prolactin) and also during illness and states of general inflammation (due to increased amounts of inflammatory molecules called cytokines). As slow wave sleep decreases with age, the amount of growth hormone secreted also declines. As our levels of growth hormone decline, there is an increase in our tendency to put on fat tissue, lose muscle mass and strength, and lose our capacity for vigorous exercise.
Q: Does the time I go to sleep make a difference to the type of sleep I have?
A: If you regularly go to sleep late and wake up late–for example, 3–11am–the proportion and cycling of REM and NREM sleep will be normal. However, if you are sleep-deprived, both REM and NREM sleep are increased.
Q: What are sleep starts?
A: On falling asleep, many people can experience a sudden jerking movement of the body that they find uncomfortable. This sensation can be associated with vivid imagery, such as a sensation of falling. This is an entirely normal phenomenon. It is called hypnic myoclonia, or sleep starts. Sleep starts can increase in frequency or intensity in times of stress or with irregular sleep-wake schedules. It is generally thought that sleep starts occur because we enter sleep through REM sleep, which is what happens in infancy.
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