What Is Sleep?  Sleep has two characteristics: -
It is reversible. A sleeping person can be awakened. -
It is the point at which you loose all consciousness of your surroundings. Your awareness of your surroundings is blocked. In the past we've referred to many things as sleep, eventhough they are not. For example, someone in a coma may be said to be sleeping, but they are not. Some people talk about death as the "long sleep", but it is not sleep. When a person falls asleep, they are instantly blind. A dramatic experiment 25 years ago showed this. Normally we have a nearly irresistable urge to fall asleep between 1am and 4am. Sometimes pilots of airliners have awakened during this time to notice everyone around them being asleep. We experience a second smaller urge to sleep between 1pm and 4pm.
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