Fatigue

We spend roughly a third of our lives asleep yet the full explanation of sleep is not entirely clear (Theories of sleep). What is clear, however, is that as we move towards sleep we become more disconnected from the real world. This, of course, has important consequences if we are controlling machinery such as a car. It might seem obvious that one ought not to fall asleep while driving a car yet in our survey of over 10,000 drivers we find that 1 in 6 drivers report that they have fallen asleep at the wheel in the last two years. As we move to a 24 hour society we can expect an increase in sleep related problems.

Theories of sleep

Restoration
Here the idea is that sleep promotes the restoration and recovery of biochemical and physiological processes. While this seems intuitively reasonable there is not a great deal of evidence
Energy conservation
During sleep there is a reduction in metabolic rate. However, it is not clear that the saving is enough warrant the natural selection of sleep.
Ecological\evolutionary
Allowed humans to avoid predators who were better prepared to hunt at night (e.g. better night vision). But this could be achieved by inactivity so why sleep.
Memory consolidation
Here the idea is that the function of sleep is to transfer information from temporary memory to long-term memory.

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