Just as people may need different size shoes they may

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Just as people may need different size shoes they may need different amounts of sleep. It is vitally important to find out how much sleep we as individuals actually need, and to then recognise that it may be different from the amount of sleep that others need. Sleeping less than we need as individuals has negative consequences. Whilst awake, we build up a sleep debt which can only be repaid through sleeping. This is regulated by a mechanism in the body called the sleep homeostat, which controls our drive to sleep. If we have a greater sleep debt, then the sleep homeostat indicates to us that we need more sleep. In a healthy situation this debt is paid off night by night. However, the debt can also build up and be repaid gradually over a period of weeks or even months, for example, if we under- sleep for several nights in a row then we will need to repay the sleep debt in the near future. Interestingly, for people with bipolar disorder, the state of mania is associated with decreased perceived need for sleep 08 . However, despite this perception, the person is still building up a sleep debt which needs to be repaid. Species Python Tiger Cat Chimpanzee Sheep African elephant Giraffe Average total sleep time per day (hours) 18 15.8 12.1 9.7 3.8 3.3 1.9 Table 1 The sleep needs of various species A mechanism called the circadian timer regulates the pattern of our sleep and waking, and interacts with the sleep homeostat. Most living things have internal circadian rhythms, meaning they are adapted to live in a cycle of day and night. The French geophysicist Jean- Jacques d’Ortous de Mairan was the first to discover circadian rhythms in an experiment with plants in 1729. Two centuries later, Dr. Nathaniel Kleitman studied the effect of circadian rhythms on human sleep cycles 09 . These rhythms respond primarily to light and darkness. The cycle is actually slightly longer than 24 hours 10 11 . It is possible to think of a “master clock” which regulates our circadian rhythms. This clock is made up of a group of nerve cells in our brain called the suprachiasmatic nucleus (SCN). The SCN controls the production of melatonin, which is a hormone that makes us feel sleepy. During sleep, melatonin levels rise sharply. The SCN is located just above our optic nerves, which send signals from the eyes to the brain. Therefore, the SCN receives information about the amount of light in the environment through our eyes. When there is less light, such as during night-time, it tells the brain to create more melatonin (see Figure 1). Light-Dark Cycle Sleep Wake Cycle Performance whilst awake Circadian Homeostat Figure 1 Diagram of sleep homeostat and circadian timer (adapted model from Professor Derk-Jan Dijk, Surrey Sleep Research Centre) Sleep Patterns Equally important as the total amount of sleep is the pattern of sleep. Babies and small children tend to sleep multiple times across each 24 hour period, but as we mature into school years and into adulthood we tend to sleep in one long phase; daytime sleeping decreases and the person instead tends to sleep throughout the night.
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  • Fall '19
  • Sleep Matters

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