What Did Ancient Humans Do All Night?

Tonight you will probably wake up two or three times. Most people do, and most people think it is a problem.

Scientists gave wrist monitors to a group of hunter-gatherers for 20 days and asked one question: when was everybody asleep at the same time? The answer was 18 minutes. Not per night. In total.

The picture in your head is wrong

You probably imagine that ancient people lay down at sunset, slept ten hours and woke up fresh. Researchers measured three groups who still live without electricity: the Hadza in Tanzania, the San in Namibia and the Tsimane in Bolivia. That was 94 adults, measured every minute.

• They slept about 6.5 hours. Less than most people in cities, and less than the eight hours you were told to get.
• They stayed up about three hours after sunset, talking in the dark.
• They fell asleep as it got cold, not as it got dark, and woke at the coldest point before dawn.

Your bedroom stays the same temperature all night. That may matter more than your screen does.

Why somebody was always awake

Night had no walls, no door and no lock. A group that all sleeps at once is in danger. You would expect guards and shifts. There were none.

People simply fall asleep at different times. Some are early sleepers, some are late, and older people sleep lightly. Spread that across 30 people and the gaps disappear. The group is guarded and nobody is on duty. This is called the sentinel idea.

So when you wake at 3 AM and think you are broken, you may just be doing your shift.

First sleep and second sleep

A historian read old court records, diaries, medical books and plays, and kept finding the same two phrases: first sleep and second sleep. He found them more than 500 times, across countries and centuries.

The pattern was this. Bed at around nine. About three hours of first sleep. Then an hour awake, on purpose. People prayed, talked, visited neighbours and made babies. Then a second sleep until morning.

The phrases fade from writing in the late 1600s, as artificial light arrived.

Two piles of evidence that disagree

The history says everybody slept twice. The wrist monitors say nobody does. The Hadza, the San and the Tsimane mostly sleep straight through.

In 1992 somebody tested it. Healthy modern men were given 14 hours of darkness every night for weeks, which is roughly a winter night before electricity. At first they slept longer. Then their sleep split in two on its own, with one to three calm hours awake in between.

That lines up with everything else. Near the equator the night is about 12 hours all year, so one sleep. In a northern winter it is 14 hours or more, so two. Same species, different night.

Both piles of evidence may be describing different nights. It is a good guess. It is still a guess.

What the night felt like

Your dark is not dark. There is a standby light, a street lamp, a phone. On a moonless night without those, you cannot see your own hand.

The fire made a room out of nothing. It also changed what people talked about. A study of the San found that daytime talk was mostly work and complaints. At night, by the fire, about 80% of it was stories: the dead, far-away people, spirits.

The short answer

They talked for three hours after dark, slept about six and a half, and fell asleep as the air cooled. Somebody was awake almost every minute, without anybody being on guard. Whether humans naturally sleep twice is not settled.

So the next time you lie awake at 3 AM, you are not broken. You are just very old.