The word "circadian" comes from the Latin phrase circa diem — meaning "about a day."
Healthy Sleep
Before electric lights existed, many people didn't sleep through the night at all — they slept twice, with an hour of quiet wakefulness in between.
Why This Matters
Getting enough sleep is something almost everyone hears about — from parents, teachers, or just a tired feeling the next day. But the science of sleep is much stranger, and much newer, than most people realize.
Researchers didn't even have a name for some of the most basic parts of sleep until the middle of the 20th century. And the way humans slept before electric light looked nothing like a modern eight-hour stretch.
The Science Story
In the 1990s, historian A. Roger Ekirch was digging through old diaries, court records, and literature for an unrelated research project when he kept running into a strange phrase: "first sleep" and "second sleep."
At first he assumed it was a mistake or a figure of speech. But the references kept showing up — in medical texts, memoirs, and even court testimony, sometimes describing a person waking during "the watch" between their two sleeps to pray, think, or visit a neighbor.
Ekirch eventually found more than 500 references to this segmented sleep pattern across centuries and cultures. Before artificial lighting made it easy to stay up late and sleep straight through the night, many people's sleep came in two separate chunks, broken up by an hour or so of quiet wakefulness.
Around the same time, other scientists were uncovering a different sleep mystery. In 1953, researchers Eugene Aserinsky and Nathaniel Kleitman noticed something odd while observing sleeping research subjects: their eyes twitched rapidly beneath closed lids at certain points in the night. They had identified what we now call REM sleep.
Did You Know?
REM sleep (rapid eye movement) wasn't identified by scientists until 1953.
Historian A. Roger Ekirch found more than 500 historical references to a two-part "first sleep" and "second sleep" pattern.
Before electric lighting spread widely, the quiet hour between first and second sleep was sometimes used for prayer, reading, or conversation.
What the Coloring Book Covers Coming Soon
The Healthy Sleep coloring book follows a wide-awake night owl exploring what happens while the rest of the house sleeps — from candlelit bedrooms of the past to the science of dreaming today. Kids color their way through a segmented-sleep bedroom scene, a nighttime sky full of circadian clues, and the sleep lab where REM sleep was first spotted.
For Parents & Educators
- If you woke up in the middle of the night the way people did centuries ago, what would you do with that quiet hour?
- Why do you think scientists didn't discover REM sleep until the 1950s, even though people have been sleeping forever?