Sleep is essential for good physical, mental and emotional health. Different people require different amounts of sleep. In general, most healthy adults need 7 - 9 hours of sleep each night. However, some people need more than 9 hours, and others can sleep less and wake up completely refreshed (contrary to myth, the need for sleep does not diminish with age, although the ability to get it all at one time may be reduced). Obviously, for each of us the amount of sleep we get is extremely important. But the type of sleep we get also determines how well-rested we will be when we awake.
The two basic types of sleep, REM (Rapid Eye Movement or dream sleep) and Non-REM (NREM), include a total of five stages that we move into and out of as we sleep. The duration and quality of these stages can vary greatly, depending on age, health, sleep hygiene, and the individual sleeper. Sleep researchers use the term “sleep architecture” to describe the cycles of sleep. Sleep usually begins with a cycle that consists of 80 minutes of NREM sleep followed by 10 minutes of REM sleep. This 90-minute cycle is repeated three to six times each night. With each cycle, the amount of slow-wave sleep decreases and the proportion of REM sleep increases. The order of a typical sleep cycle is: waking, stage 1, 2, 3, 4, 3, 2, 5 (REM).
About 80% of adult sleep is NREM sleep. NREM sleep is divided into four stages:
Stage 1 (transitional sleep): The transitional stage is the drowsy period from wakefulness to deeper sleep, and usually lasts less than 15 minutes. This is the lightest period of sleep and during it we are more responsive to sounds and external activities and are more easily awakened. Brain waves, as measured by an electroencephalogram (EEG), are fast and are known as "beta" waves. During this transitional stage breathing becomes slow and regular, heart rate decreases, and the eyes exhibit slow rolling movements.
Stage 2 (light sleep): This is a deeper stage of sleep and lasts 15 to 30 minutes in each sleep cycle (over the whole of a night it makes up 45 – 55% of our total sleep). Here, the brain waves, now called "theta" waves, slow even further and fragmented thoughts and images pass through the mind. Eye movements usually disappear, muscles relax, and there is very little body movement.
Stages 3 and 4 (deep sleep): Stages 3 and 4 are often referred to as deep sleep or “delta” sleep. These stages normally last for 30 to 40 minutes in the first sleep cycle and decrease with subsequent cycles throughout the night. In stages 3 and 4 there is little contact with external sensation, breathing slows down even more, muscles relax, and the heart beats slower and slower. Dreams are more common during stages 3 and 4 of sleep, and people are more likely to talk to themselves. Deep sleep decreases with age - by age 75, stage 4, the deepest sleep, may be completely absent.
About 20% of sleep is REM, or dream sleep.
Stage 5 (REM or dream sleep): REM sleep typically occurs at the end of each sleep cycle - though some researchers have noted the presence of dream activity during NREM sleep as well. Unlike NREM sleep, REM sleep involves a high level of mental and physical activity, including increased brain activity, blood pressure, heart rate, blood flow to the brain and respiration. Intense dreaming occurs during REM sleep as a result of heightened cerebral activity, but interestingly, REM sleep is also characterized by muscular immobility or paralysis. It is generally thought that REM-associated muscle paralysis is meant to keep the body from acting out the dreams that occur during this intense sleep stage. The first period of REM sleep typically lasts 10 minutes, with each recurring REM stage lengthening, and the final one lasting an hour. In the newborn baby, more than 50% of sleep may be REM sleep. By the age of 2 years, the proportion of REM sleep decreases to 20% to 25% and remains constant throughout adulthood.