Sleep can be looked at in a number of different ways. Firstly, by considering level of consciousness. The sleeper has reduced awareness and can be roused by environmental stimuli such as loud noise or light. This is clearly very different to a state of unconsciousness.We are most sensitive to sound stimuli when we sleep as the bed partners of snorers will testify!
Another way to consider sleep is to look at the activity of the brain. Sleep is by definition a primary activity of the brain. During sleep some parts of the brain are active and some parts - for example much of the pre-frontal cortex (which controls sophisticated behaviours such as self-control and reasoning) is resting. The reasons for this are still being explored but it may be that sleep allows some parts of the brain to ‘recharge’.
The traditional way to look at sleep is by measuring brain activity with small sensors applied painlessly to the scalp. This is part of a standard sleep study. This allows us to study the very different phases of brain activity that comprise sleep. Much of the first part of the night is spent in slow wave (or deep) sleep whilst rapid eye movement (REM) sleep is more common in the second half of the night. Certain sleep disorders are associated with particular sleep stages - in this way a sleep study can help make an accurate diagnosis of a sleep disorder. Some prescription medication and alcohol can affect our sleep stages and thus interfere with sleep quality.
Why we need to sleep
Sleep requirements vary through life, but children and adolescents need more than adults. Preschoolers require 11 to 13 hours a day and most have an afternoon nap. School age children need 10 to 11 hours of sleep. Sleep patterns change in adolescence so that bed-time is naturally later and young people may be reluctant to wake in the morning. The average adults needs 7-8 hours sleep out of 24 hours. In some cultures these sleeps are divided into a shorter night's sleep and a siesta. Most Northern Europeans however take their sleep during a single phase at night. As we age it is harder to sleep and increasingly easy to nap in the day!
How much sleep do we need?
Sleep requirements vary through life, but children and adolescents need more than adults. Preschoolers require 11 to 13 hours a day and most have an afternoon nap. School age children need 10 to 11 hours of sleep. Sleep patterns change in adolescence so that bed-time is naturally later and young people may be reluctant to wake in the morning. The average adults needs 7-8 hours sleep out of 24 hours. In some cultures these sleeps are divided into a shorter night's sleep and a siesta. Most Northern Europeans however take their sleep during a single phase at night. As we age it is harder to sleep and increasingly easy to nap in the day!
What controls when we sleep?
We all have a biological clock located deep within the brain in a structure called the hypothalamus. This controls our desire to sleep. As long as we stay in one time zone we are naturally most sleepy at around 3pm and 3am. It is possible to reset your biological clock to new time zones and this is helped particularly to exposure to natural light in the new zone although naturally takes several days . This is why we feel tired at inappropriate times of day when we change time zones. However, our sleep is not simply controlled by our biological clock. Other factors are really important. To fall asleep we need to feel safe and comfortable, have a quiet dark environment and be free from pain and anxiety. Most of us prefer to sleep lying down. If any of these factors are altered the chances are we will struggle to fall asleep. However most importantly we need to feel tired and even in an optimal sleep environment most adults would not fall asleep readily in the morning after a good night’s sleep. Thus we see that sleep is driven by how tired we are, our biological clock as well as having the right conditions to promote sleep.