Continuous sleep is a modern habit, not an evolutionary constant, which helps explain why many of us still wake at 3 am and wonder if something's wrong. It might help to know that this is a deeply human experience. For most of human history, a continuous eight-hour snooze was not the norm. Instead, people commonly slept in two shifts each night, often called a "first sleep" and "second sleep." Each of these sleeps lasted several hours, separated by a gap of wakefulness for an hour or more in