Considering that we spend 1/3 of our lives asleep, we give it very little thought. The importance of sleep is regularly overlooked from day to day and not given the same attention as, for example, our diet and exercise plans. Sleep patterns changed massively with the invention of electric lights, and have only worsened since, as technology allows 24-hour communications. It is not exactly surprising that this sleep deprivation has been found to have many negative consequences.
Facts about sleep
o Our liver, heart, pancreas, kidneys and lungs are all able to create their own circadian rhythms, independent of the brain. Altering meal times and exposure to light can cause these factors to be out of sync with each other.
o Humans sleep around 3 hours less per night than most primates.
o The shift to daylight saving time in spring (causing one hour less sleep) results in nearly 20% more road accidents on the following Monday.
o 40-70% of alcoholics suffer from insomnia.
o Twice as many people are killed or injured by drowsy-driving as are by driving accidents related to drugs.
o After 24 hours without sleep, performance is impaired so much that levels fall to those of a drunk person (blood alcohol greater than 0.08%).
o When we are awake, adenosine builds up, causing us to feel drowsy after a long period without sleep. Caffeine blocks the adenosine receptors in the central nervous system as well as increasing the heart rate.
o It takes around 5 hours for half of the caffeine in our body to be metabolised (the half life). The way we respond to caffeine varies between individuals due to differences such as in the adenosine A2A receptor gene, altering the level of sleep disruption caused.
o Our circadian clock creates an almost 24 hour rhythm which causes us to feel tired at almost the same time each day. We rely on environmental cues, such as light, to reset this every day because it may be a little under or over 24 hours. It is therefore very common for blind people to suffer from non 24-hour sleep-wake disorder, going through phases of good and bad sleep as their rhythm moves in and out of sync with day and night times.
Stages of Sleep
In 1928, the electroencephalogram (EEG) was first used, allowing the study of voltage changes in the brain. These changes are caused by neurone signals and therefore it became possible to track brain activity. Studying this has given us a much greater insight into what happens during sleep.
Sleep occurs in different stages, that take place in a set order (the sleep cycle). This cycle takes around 90 minutes and is then repeated. It consists of non-rapid eye movement sleep (NREM) and rapid eye movement sleep (REM). There are 4 stages of NREM sleep, during which conscious awareness of our surroundings disappears and, in the final 2 stages, we may begin dreaming. Activities such as walking and talking also can occur in the final 2 stages of this NREM sleep. This takes around 70 minutes and then REM sleep takes place, involving similar patterns of brain activity to being awake. Dreaming within this phase of sleep is likely to be more vivid and complex.
Importance of Sleep
The initial reason why sleep was evolutionary favourable is unknown. There are 3 main theories to explain this:
-> for cellular restoration
-> for energy conservation
-> for memory consolidation
Whatever advantages sleep initially brought to animals, we now understand that sleep has a great many benefits.
The National Sleep Foundation recommend adults sleep for 7-9 hours per night. It is fairly well agreed that around 8 hours of sleep are required for an adult, and between 9-10 hours for a teenager. the National Health Interview Survey found that from 2005-2007, nearly 30% of adults had 6 hours or less sleep per day.
This sleep deprivation is worrying as it is associated with many health problems:
The reason why sleep deficiency increases the risk of CVD is unknown, but there are several possibilities. Metabolic changes occur during sleep which could mean that a deficiency increases the level of fat in the blood and platelets are most able to form clots throughout the nighttime. The endothelial cells of blood vessels, hormone levels, heart rate and cytokine production are also affected by sleep.
During sleep, our blood sugar level is maintained at a constant level. When we are awake throughout the night, this level will fall and this affects our insulin sensitivity. Studies have shown that after sleep deprivation, insulin is less effective at lowering blood glucose. there are other factors linking sleep and diabetes as well, such as an increased carbohydrate intake.
-Anxiety, depression and other mental health
There is a strong link between sleep deprivation and poor mental health, with either one being a risk factor for the other. For example, 80% of people with depression or schizophrenia have sleep problems. It has also been suggested that having a history of insomnia increases the risk of depression by 4 times. The way in which sleep affects mental health is not fully understood but it is known to bring about several hormonal changes which could play a part.
-Decreased cognitive performance, memory and decision making
Through the use of EEG, it has been observed that often during sleep, the same neural activity is repeated as took place when awake. This may indicate the formation of memory by replaying the day’s events. Many studies have also found memory to be better in subjects allowed a greater number of hours asleep.
The link between sleep and cognitive performance is particularly noticeable in adolescents, who experience a 2-3 hour difference in circadian rhythm compared to adults, which occurs naturally. This is not a result of using technology throughout the night, even though this can worsen sleep patterns. This means that an adolescent waking up at 6 or 7 am is equivalent to an adult waking up at 3 or 4 am.
A US study showed that delaying the start of school by 1 hour reduced the number of road accidents in 17-18 year olds by 16.5%. The same study found (as lots of others have) that there was a positive correlation between numbers of hours slept and grades achieved by the individuals. By moving the school start time forward, many people expected the bedtimes of students to become later, however they remained constant throughout the experiment, resulting in longer nights of sleep.
It has been shown that after sleep deprivation, appetite will increase due to hormone changes. This comes with a particular craving for carbohydrates. The hormone Leptin is produced by fat cells in the body and causes us to feel satisfied, where as Ghrelin is mainly produced by the stomach to stimulate appetite. When deprived of sleep, our leptin production is reduced as well as our Ghrelin production being increased. Eating at night is also dangerous for dieters as it has been shown that the amount of insulin, glucose and fat in the blood will be at completely different levels depending on whether a meal is eaten during the day or night.
-Poor immune response
In 2002, a study in the Journal of the American Medical Association gave 2 groups of subjects an influenza immunisation. One group had slept for 4 hours and the other for 8 hours. The findings were that the group with less sleep had produces less than half of the amount of antibody as the other group. A 2012 study also observed the number of white blood cells decreasing throughout 29 hours of being awake.
-Types of cancer
The link between cancer and lack of sleep could be attributed to changes to hormone levels, reduced melatonin levels, reduced immune function or disruption of the cell cycle. Whatever the cause, there is evidence that a link does exist. Shift workers with extreme disruption of their circadian rhythm have been found to have a 50% greater risk of breast or prostate cancer. Links have also been found with other types of cancer and the risk varies with the level of sleep disruption.
Journal of Clinical Sleep Medicine
National Sleep Foundation
The National Sleep Research Project
Sleep – A Very Short Introduction (Oxford University Press)
Centre for Disease Control and Prevention
The Better Sleep Council
American Psychological Association
Science Direct: The Metabolic Consequences of Sleep Deprivation
Journal of Applied Physiology – Sleep Loss: A novel risk factor for insulin resistance and Type 2 diabetes
Principles and Practise of Sleep Medicine sample
ScienceDaily: sleep deprivation effect on the immune system mirrors physical stress
National Institutes of Health
Harvard Health Publications
Cancer Treatment Centres of America