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 the middle of the night. Historical records from Europe, Africa, Asia, and beyond describe how, after nightfall, families would go to bed early, then wake around midnight for a while before returning to sleep until dawn.

Breaking the night into two parts probably changed how time felt. The quiet interval gave nights a clear middle, which can make long winter evenings feel less continuous and easier to manage.

The midnight interval was not dead time; it was noticed time, which shapes how long nights are experienced.

Some people would get up to tend to chores like stirring the fire or checking on animals. Others stayed in bed to pray or contemplate dreams they'd just had. Letters and diaries from pre-industrial times mention people using the quiet hours to read, write, or even socialise quietly with family or neighbours. Many couples took advantage of this midnight wakefulness for intimacy.

Literature from as far back as ancient Greek poet Homer and Roman poet Virgil contains references to an "hour which terminates the first sleep," indicating how commonplace the two-shift night was.

How we lost the 'second sleep'
The disappearance of the second sleep happened over the past two centuries due to profound societal changes.

Artificial lighting is one of them. In the 1700s and 1800s, first oil lamps, then gas lighting, and eventually electric light, began turning night into more usable waking time. Instead of going to bed shortly after sunset, people started staying up later into the evening under lamplight.