07/14/2026
Maybe we don’t need to stress about waking up in the middle of the night.
Historical records from the 1500s and 1600s describe a "first sleep" and a "second sleep," with a natural gap in between where people got up in the middle of the night. Historian A. Roger Ekirch has pulled more than 500 references to this pattern from ancient, medieval, and early-modern documents. The waking stretch even had a name: "the watch."
Sleep scientists call the two-block pattern biphasic or segmented sleep. The broader term polyphasic, coined in the early 1900s by psychologist J. S. Szymanski, covers any schedule with multiple sleep periods in 24 hours, and it's believed to be the ancestral state for mammals.
Ekirch attributes the shift to a single unbroken block to the spread of electric lighting. A 1992 study by Thomas Wehr backs the older rhythm: healthy men kept in 14 hours of darkness each night eventually settled into roughly 4 hours of sleep, 2 to 3 hours awake, then another 4 hours of sleep. During that middle stretch, the brain runs high on prolactin, the pituitary hormone tied to the calm, floaty feeling people often report at that hour.
The single unbroken block we treat as normal is the newer habit. If you jolt awake at 2AM and can't explain why, your body might just be running an older pattern.
