Sleep: The secret to sleeping well and waking refreshed. Prof. Idzikowski Chris
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Название: Sleep: The secret to sleeping well and waking refreshed
Автор: Prof. Idzikowski Chris
Издательство: HarperCollins
Жанр: Спорт, фитнес
isbn: 9780007362493
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24-hour cycle
Most living organisms, plants and animals, live according to a 24-hour cycle that is dominated by light and darkness. Even death can be part of this cycle, with cardiac arrests and strokes occurring mainly between 6 a.m. and 12 noon – perhaps because this is when blood tends to clot most.
Clocks, cycles and rhythms
We are all governed by a 24-hour cycle called a ‘circadian rhythm’, taken from the Latin words circa, meaning ‘around’, and die, meaning ‘day’. Circadian rhythms underpin everything, from hormone production to when we feel like getting up or going to bed. Our body temperature has a 24-hour rhythm too; minimum body temperature usually occurs around 4 a.m., maximum body temperature around 10-11 p.m. Sleep also roughly follows a 24-hour rhythm.
For most of us, a typical cycle means falling asleep between around 11 p.m. and midnight, and waking up between 6 a.m. and 8 a.m., indicating that we are biologically programmed to be able to fall asleep and wake up at around those times. However, not all clocks keep the correct time, and the biological clock is no exception. It generally runs a little ‘slow’ but is kept to the right time relative to light and darkness by ‘synchronizing’ cues called ‘zeitgebers’. Dawn light is one of the most important and well-understood cues that our body responds to, but the onset of darkness (which stimulates the production of melatonin from the pineal gland) also has a role to play. Other zeitgebers are exercise, mealtimes, social interactions, sounds, and possibly changes in temperature. Sleep itself may be a weak zeitgeber.
Sleep and wakefulness
Working with the ebb and flow of the circadian rhythm are special sleep and wakefulness centres, which are located in a part of the brain called the hypothalamus. The sleep centre is in the same region of the brain that controls temperature (which may be why you sometimes can’t sleep if you are too hot) and the wakefulness centre is near the part that is associated with activity. In an ideal world, the sleep centre will shut down during the day, when the wakefulness centre opens, and open at night, when the wakefulness centre closes. Not surprisingly, good sleepers have strong day-time wakefulness and night-time sleep systems. But if these centres have been damaged (through, say, over-use of caffeine, alcohol or drugs, or due to illness or age), you are likely to have sleeping problems.
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Sleep vs. wakefulness
The discovery of sleep and wakefulness centres in the brain came about as the result of the spread of a brain disease called Encephalitis lethargica between 1917 and 1928. The disease, characterized by a lethargy that turns its sufferers into living statues, became the subject of the Oliver Sachs book Awakenings (1973).
The brain’s sleep centres
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Cues
The brain’s metronome or clock runs slowly. Various cues such as light, exercise and food intake keep it synchronized with day and night.
The brain’s metronome
The suprachiasmatic nucleus (SCN), is the brain’s master biological clock, or metronome. It helps to synchronize sleep with the circadian rhythm, as well as regulating many of the bodily functions that affect sleep, such as hormonal secretion and body temperature. Made up of about 20,000 nerve cells, the SCN is located in the brain’s hypothalamus, just behind the eyes, and is immensely important; if it is destroyed, 24-hour rhythms break down. It keeps time with daylight and is believed to work particularly closely with the wakefulness centre of the brain. Scientists also believe it ‘instructs’ the pineal gland to release melatonin, the hormone that signals the onset of darkness.
In 2002, a group of scientists discovered that the SCN is directly connected to a receptor in the retina sensitive to blue light (light from the sky), which enables the brain to identify whether it is light or dark. In blind people, who have damaged retinas, the SCN does not synchronize with daylight and so their ability to sleep is affected.
The role of melatonin
Melatonin is secreted by the pineal gland in the brain (often known as the ‘third eye’ in reptiles because of its sensitivity to light), and is the hormone that ‘tells’ the brain it is dark. Secretion starts when it gets darker, peaks in the middle of the night and stops at dawn. Melatonin is believed to be an important synchronizer of other circadian rhythms and is closely connected with Seasonal Affective Disorder (SAD) and jet lag.
Seasonal Affective Disorder (SAD)
Seasonal affective disorder, or SAD, is a condition in which people suffer depression, insomnia and lethargy in the winter months – hence its alternative name, ‘winter depression’. Normal circadian rhythms dictate that we get up in the light and go to bed in the dark. This is fine in the summer, but, in the case of SAD sufferers, the lack of light in winter is believed to disrupt their body clock so much that getting up on dark winter mornings proves impossible. The problem is believed to be caused by a disruption in their production levels of melatonin, the hormone which acts as the brain’s and body’s signal for darkness. Melatonin is normally produced at night, but in 80 per cent of SAD sufferers, melatonin levels peak just when it’s time to get up. SAD is often treated with light therapy, in which sufferers are subjected to bright light. The light is believed to block the production of melatonin, which kickstarts and resets the body clock.
The role of serotonin
Serotonin as a precursor of melatonin is one of the brain’s chemical messengers and has a major effect on the way the brain works, specifically affecting mood and sleep. (Other chemicals in this group include noradrenaline, dopamine and histamine.) It is produced in the brain from trytophan, an essential amino acid found in certain foods, and levels in the body are negatively affected by poor diet and stress. Lack of serotonin may lead not only to insomnia, but to anxiety and depression, which are in turn two of the greatest disruptors of sleep. Often known as the ‘feel good’ hormone, serotonin is Nature’s own Prozac, and is a crucial in the way many of the more recent antidepressant drugs, known as Selective Serotonin Re-uptake Inhibitors (SSRIs), work.
did you know?
• Serotonin is one of the oldest brain chemicals around. Serotonin neurones existed in animals that appeared on earth 500 million years ago.
• Women produce up to a third less serotonin than men.
What happens when we sleep
Doctors and scientists can now record the activity of the brain in sleep by means of an electro-encephalogram or EEG. Using EEGs in this way has revolutionized our understanding of sleep.
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