Sleep is needed for the body to rest and repair.
SLEEP is something none of us can do without. However, the reason why everyone needs sleep is not well-elucidated. During sleep, the brain is more responsive to internal stimuli than external stimuli like sound and light. The metabolic demand of the brain is also reduced and this is reflected in a general decrease in the blood flow to the brain.
Most of the knowledge about sleep has come from sleep deprivation experiments. It is generally accepted that good sleep is crucial for optimal intellectual performance and helps in realising a person’s mental potential.
Normal sleep comprises of cycles of non-rapid eye movement (NREM) and rapid eye movement (REM) sleep. NREM sleep consists of progressively deeper stages of sleep. As NREM sleep progresses, stronger stimuli are needed for awakening. REM sleep, which is also known as “dream sleep”, has tonic and phasic components.
There are no rapid eye movements in the former. There are rapid eye movements, muscle twitches, pupil dilatation, increased heart rate variability and increased breathing rate in the latter. The muscle tone is decreased throughout REM sleep. The length of REM sleep and the intensity of the eye movements increase throughout the sleep cycle.
NREM sleep is followed by REM sleep, which occurs four to five times during the usual eight hour sleep period. The first REM period of the night may be last less than 10 minutes, while the last may exceed an hour. The NREM and REM cycles vary in length from 70 to 100 minutes initially to 90 to 120 minutes later in the night.
During the first third of the night, deep NREM sleep is present more whereas REM sleep predominates in the last third of the night. REM sleep takes up 20 to 25% of total sleep time. Light NREM sleep is the transition between sleep and awakening.
Infants sleep more than any other age group. The newborn baby can sleep 14 to 16 hours in a day. As the baby grows, the sleep time decreases so that by about six months of age, there is usually an overnight sleep period with at least a nap during the day. There is more REM sleep in an infant.
Senior citizens take a longer time to fall asleep and have more frequent awakenings. This sleep fragmentation may be aggravated by medical problems. There is less deep sleep and the total time in bed may increase leading to complaints of insomnia.
How much sleep?
The need for sleep varies with age. A newborn may sleep 16 to 20 hours throughout a day and an infant 12 to 14 hours with most of the sleep at night. Toddlers may sleep 10 hours or more. Primary school children need nine to 10 hours of sleep. Normal adults need six to 10 hours of sleep. It usually takes an adult about 10 to 20 minutes to fall asleep. Most of those who have less than five to six hours of sleep are probably not getting enough sleep.
After a good sleep, a person would feel refreshed on waking and is able to be alert throughout the day, without the need for naps or sleeping in on weekends.
The body has a “biological clock” called the circadian rhythm which is set by the hypothalamus in the brain. It is set at about 24.2 hours and the body is used to a regular routine of light and darkness at certain times. Light is called zeitgeber, a German word meaning time-giver, because it sets the clock in the hypothalamus.
A practical explanation for the circadian rhythm is that the brain is analogous to a battery charging during sleep and discharging during the time a person is awake.
It is believed there is a downswing in the circadian rhythm from evening till it reaches the lowest point (nadir) in the early morning.This makes it possible for a person to remain asleep overnight by preventing premature awakening, which is facilitated by the morning upswing. The upswing peaks in the early evening after which there is a downswing. This explains the stable cognitive function during the time a person is awake.
The body temperature is controlled by the hypothalamus. It is increased during the daylight hours and decreased at night, thus mirroring the sleep rhythm. Melatonin, prolactin, testosterone and growth hormone also have a circadian rhythm with maximal secretion during the night.
Much knowledge about sleep has come from sleep deprivation experiments. Studies of individuals deprived of sleep for more than 24 hours have shown that there is a decrease in the brain’s metabolic activity by up to 6% for the whole brain and 11% for specific areas in the brain. There is also a decrease in body temperature, release of growth hormone and immunity as well as an increase in heart rate variability.
As sleep has a restorative function, deprivation leads to short and long term serious consequences. There are significant effects of sleep deprivation on brain functions like memory, concentration and mood.
Higher order cognitive function like language and numerical skills is affected early and disproportionately. There may be memory lapses, blurring of vision, slurring of speech, disorientation and poor co-ordination of body movements. Mood changes like depression are common.
Short term sleep deprivation has been reported to contribute to obesity and poor control of type II diabetes. Chronic partial sleep deprivation is the commonest cause of daytime sleepiness, which can impair performance at school or work and increases the risk of accidents especially when driving or operating machinery.
Long term sleep deprivation decreases the quality of life and leads to increased morbidity and mortality. There are effects on cardiovascular, respiratory and endocrine function. Coronary events have been found to be more common in those who have less than seven hours sleep compared to those with eight hours or more.
The subtle cognitive changes with small amounts of sleep loss (less than an hour each night for many nights) may not be recognised by the affected individual. Sleep loss for a week leads to marked cognitive deficits which may also be unrecognised by the affected individual. Research and education is ongoing to address this lack of recognition of the effects of sleep deprivation.
Getting a good night’s sleep is vital for everyone. There are several ways of achieving this and they include:
- Having a routine facilitates sound sleep. This means going to bed and getting up at about the same time every day.
- Having an early dinner is helpful. The digestive system goes to sleep at about 7pm. A light dinner is helpful.
- Avoiding caffeine after lunch is helpful as caffeine keeps one awake.
- Avoiding alcohol is helpful as its breakdown produces chemicals that stimulate the individual. It also increases the likelihood of snoring as it relaxes the muscles. This leads to lighter and less refreshing sleep.
- Avoiding naps. The afternoon nap may be what keeps one awake at night.
- Avoiding light can be helpful. Not going to bed immediately after turning off the television. This is because melatonin, which is the hormone that helps a person sleep, is produced in the dark.
Source: Dr Milton Lum