What is the Sleep Cycle?
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The human body needs sleep to function properly and stay healthy. Some may think of sleep as a passive state where the brain turns off to rest, but this isn’t true. The brain stays active during sleep and may even be more active than when awake. This brain activity is broken up into several phases that, together, make up the sleep cycle. One cycle takes between one and two hours to complete and consists of rapid-eye-movement and non-rapid-eye-movement phases.
By using electroencephalography to view brain waves, scientists and researchers studying sleep discovered that brain activity varies throughout the night. Delta wave activity seems particularly variable. Certain bodily activities also change alongside the delta wave oscillation. When the delta wave activity increases during NREM sleep, secretion of the hormones responsible for lactation, blood pressure, and growth increase as well. During periods of decreased delta wave activity, such as the REM phase, thyroid-stimulating hormone secretions and heart rate variability increase.
The REM phase is sometimes referred to as paradoxical or active sleep. Frequent eye movement characterizes this phase, which is the easiest to wake from and is also the period when dreaming occurs. During this phase, respiration can accelerate, and brain activity can increase, similar to being awake. The muscles in the arms and legs become less responsive or even completely paralyzed during this phase, which researchers believe is a way to protect the body from harming itself during dreams. The longer we sleep, the more periods of REM phase we experience.
Most healthy adults begin the sleep cycle with NREM sleep. This type of sleep is broken down into stages: N1, N2, and N3. From N1 to N3, the brain becomes increasingly less active, and the eyes remain completely still. The large and slow waves measured during this period led to its other name -- delta or deep sleep. The number of dreams reported during periods of NREM sleep are much lower than the REM phase. Generally, the body will ease into the stages and then fluctuate between N2, N3, and REM sleep.
The NREM sleep period begins with the N1 stage. This occurs between seconds and minutes of the body falling asleep and lasts for up to seven minutes. From there, the cycle moves into N2. The measured brain waves first appear to slow down and increase in size during the N2 stage, usually reaching what is referred to as deep sleep. After the N2 stage is the N3 stage which is the period of deepest sleep. The N3 stage becomes progressively shorter in each cycle, and may disappear entirely during long periods of rest. The cycle will usually rotate from N2 to N3 with a brief return to the N2 stage before entering the REM phase. After the REM period, the body shifts back into the N2 stage.
Sleep Cycle Length
In infants and young children, the sleep cycle is significantly shorter than in adults. The first cycle, marked by a full rotation through NREM and REM sleep, lasts around 50 minutes to an hour and this length steadily increases with age. In adulthood, the first sleep cycle usually takes between 70 and 100 minutes. Each subsequent cycle lasts around 90 to 120 minutes. A typical seven to eight hour sleeping period contains five cycles, with the middle two taking a bit longer to complete than the others.
Changes in the Cycle
Many variables can alter sleep patterns. Age, time of day, exercise, stress, and even quality of past sleep can affect sleep quality. Irregular or poor sleep schedules can result in prolonged periods of NREM sleep so the body can spend more time repairing itself with hormone secretions. Drugs and alcohol can affect the REM period, shortening or eliminating it entirely. The body eventually metabolizes the substances, and the REM phase returns, but there may be a marked increase in fatigue or tiredness the following day.
The Effects of Age
One of the biggest impacts on sleep cycle is age. Along with increasing the overall length of the sleep cycle, age also affects the individual stages and phases of the cycle. The deep sleep periods, N2 and N3, continue to decrease, so older adults produce fewer regenerative hormone secretions during sleep. Because they spend less time in the NREM stages, people is more likely to be easily awakened as they age. Reductions in sleep duration and difficulty getting to sleep are also more common.
In some areas of the world, most people opt to sleep in undisturbed eight-hour blocks. However, many other cultures choose to take naps during the daytime. Naps during the day can interrupt the constantly increasing signal that tells the body when it needs to sleep, which can lead to increased alertness and activity. Taking a nap longer than a single cycle or taking a nap while suffering from insomnia may lead to increased difficulty sleeping at night.
Lack of Sleep
Sleep deprivation can be seriously debilitating and cause physical and mental issues. The most common symptoms include daytime drowsiness and periods of microsleep, where the body sleeps briefly despite being awake. A sleep-deprived person may also experience irritability, depression, and anxiety. Prolonged lack of sleep can lead to complications such as diabetes, high blood pressure, weight gain, and heart disease. Sleep deprivation causes thousands of vehicle accidents each year.
Many life changes can improve his or her sleep quality. For those experiencing severe sleep issues, one easy solution is to increase exposure to direct sunlight. In studies, older people and those with insomnia often find a mere two hours of sunlight dramatically increase sleep quality, as does lengthening the duration of sleep. Research also shows that consistent waking and sleeping times are best.
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