Karen Emslie tells NPR listeners about segmented sleep, which she says was the norm before electric lights were commonplace.

KAREN EMSLIE: So people went to bed earlier and naturally woke up after a few hours of sleep and had a period of night waking of perhaps two to three hours, which they used for quite peaceful activities like reading and writing and praying or making love, as they were refreshed after their first sleep. Then they would go back to sleep and then wake up naturally in the morning.

VIGELAND: This sleeping pattern is called segmented sleep. Historical documents across cultures show plenty of references to a first and second sleep, divided by a period of being awake in the middle of the night. But the light bulb changed everything.