Passing through non-REM and then REM sleep adds up to a sleep cycle. A great deal gets done during such cycles, and it appears that we need five or six of them to reach full restoration. How do you know whether you’re getting enough sleep? Unless you’ve got certain medical conditions or are taking medications that cause fatigue, a simple rule of thumb is that if you can’t wake up in the morning without an alarm clock and you feel sleepy before lunch, either you are sleep-deprived or, as we saw in Chapter 8, your circadian clock is misaligned. To determine your personal amount of sleep needed,
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