Why Did Ancient Humans Sleep in Two Parts?

You wake at 3 in the morning and think your sleep is broken. For most of history, it was not broken — it was normal. Historian A. Roger Ekirch found that pre-industrial Europeans slept in two parts: a first sleep, a waking hour or two, then a second sleep. In 1992, Thomas Wehr's laboratory found the same pattern when volunteers lived under long winter nights. But living hunter-gatherers studied by Yetish and colleagues in 2015 slept in one block — so the honest answer is more interesting than the simple story. This video follows the evidence, including the awkward fact, and ends with a different way to see tonight.