We spend around a third of our live sleeping; it is vital to our survival, but despite years of research, scientists still aren’t entirely sure why we do it. The urge to sleep is all- consuming, and if we are deprived of it, we will eventually slip into slumber even if the situation is life- threatening.
sleep is common to mammals, birds and reptiles and has been conserved through evolution, even through evolution, even though it prevents us from performing tasks such as eating, reproducing and raising young. It is as important as food for keeping us alive; without it, rate will die within two or three weeks- the same amount of time it takes to die of starvation.
There have been many ideas and theories about why we sleep, from a way to rest after the day’s activities or a method for saving energy, to simply a way to fill time until we can be doing something useful, but all of these ideas are somewhat flawed. The body repairs itself just as well when we are sitting quietly, we only save around 100 calories a night by sleeping, and we wouldn’t need to catch up on sleep during the day if it were just to fill empty time at night.
One of the major problems with sleep deprivation is a resulting decline in cognitive ability; our brains just don’t work properly without sleep. We struggle with memory, learning, planning and reasoning. A lack of sleep can have severe effects on our performance, ranging from irritability and low mood, through to an increased risk of heart disease and higher and higher incidence of road traffic accidents.
Sleep can be divided into two broad stages, non- rapid eye movement (NREM) sleep. The vast majority of our sleep (around 75 to 80 per cent) is NREM, characterized by electrical patterns in the brain known as ‘sleep spindles’ and high, slow delta waves. This is the time we sleep the deepest.
Without NREM sleep, our ability to form declaration memories, such as learning to associate pairs of words, can be seriously impaired; deep sleep is important for transferring short- term memories into long- term storage. Deep sleep is also the time of peak growth hormone release in the body, which is important for cell reproduction and repair.
The purpose of REM sleep is unclear; the effects of REM sleep deprivation are less severe than NREM deprivation, and for the first two weeks humans report little in the way of ill effects.
REM sleep is the period during the night when we have our most vivid dreams, but people dream during both NREM and REM sleep. During NREM sleep, dreams tend to be more concept- base, whereas during REM sleep dreams are more vivid and emotional.
Some scientists argue that REM sleep allows brains a safe place to practice dealing with situations or emotions that not encounter during our daily lives; during REM sleep our muscles are temporarily paralyzed, preventing us acting out these emotions. Others think that it might be a way to unlearn memories, or to process unwanted feelings or emotions. Each of these ideas has its flaws, and no one knows the real answer.
During the night, you cycle through five separates stages of sleep every 90 to 110 minutes.
The five stages of sleep can be distinguished by changes in the electrical activity in your brain, measured by electroencephalogram (EEG). The first stage begins with drowsiness as you drift in and out of consciousness, and is followed by light sleep and then by two stages of deep sleep. Your brain activity starts to slow down, your breathing, heart rate and temperature drop, and become progressively more difficult to wake up. Finally, your brain perks up again, resuming activity that looks much more like wakefulness, and you enter rapid eye movement (REM) sleep; the time that your most vivid dreams occur. This cycle happens several times throughout the night, and each time, the period of REM sleep grows longer.
During the first stage you are just drifting off; your eyelids are heavy and your head starts to drop. During this drowsy period, you are easily awoken and your brain is still quite active. The electrical activity on an electroencephalogram (EEG) monitor starts to slow down, and the cortical waves become taller and spikier. As the sleep cycle repeats during the night, you re- enter this drowsy half- awake, half- sleep stage.
After a few minutes, your brain activity slows further, and you descend into light sleep. On the EEG monitor, this stage is characterized b further slowing in the waves with an increase in their size, and short one or two- second bursts of activity known as ‘sleep spindles’. By the time you are in the second phase of sleep, your eyes stop moving, but you can still be woken quite easily.
As you start to enter this stage, your sleep spindles stop, showing that your brain has entered moderate sleep. This is then followed by deep sleep. The trace on the EEG slows still further as your brain produces delta waves with occasional spikes of smaller faster waves in between. As you progress through stage- three sleep, you become much more difficult to wake up.
There is some debate as to whether sleep stages three and four are really separate, or whether they are part of the same phase of sleep. Stage four is the deepest stage, and during this time, you are extremely hard to wake. The EEG shows tall, slow waves known as delta waves, your muscles relax and your breathing becomes slow and rhythmic, which can lead to snoring.
After deep sleep, your brain starts to perk up and its electrical activity starts to resemble the waking brain. This is the period of the night when most dreams happens. Your muscles are temporarily paralyzed, and your eyes dart back and forth, giving its name, rapid eye movement (REM) sleep. You cycle through the stages of sleep about every 90 minutes, experiencing between three and five dream periods each night.
We save around 100 calories per night by sleeping; metabolic rate drops, the digestive system is less active, heart and breathing rates slow, heart and breathing rates slow, and body temperature drops. However, the calories saving equates to just one cup of milk, which from an evolutionary perspective does not seem worth the accompanying vulnerability.
An early idea about the purpose of sleep is that it is a protective adaptation to fill time. For example, prey animals with night vision might sleep during the day to avoid being spotted by predators. However, this theory cannot explain why sleep- deprived people fall asleep in the middle of the day.
One of the major problems with sleep deprivation is decrease in cognitive function, accompanied by a drop in mood, and there us mounting evidence that sleep is involved in restoring the brain. However, there is little evidence to suggest that the body undergoes more repair during sleep compared to rest or relaxation.
One of the strongest theories regarding sleep is that it help with consolidation of memory. The brain is bombarded with more information during the day than it is possible to remember, so sleep is used to sort through this information and selectively practice parts that need to be stored.