I just assumed that most people knew what second sleep was. I learned years ago when I was curious about Edison's crazy sleep habits as I've always had sleep issues. I remember hearing the living room clock dong on each hour then falling asleep at around 1 only to get up at 6 for work or school.
Second sleep was universally the norm:
Understanding the Two-Sleep Cycle
Biphasic Sleep: Also known as segmented or bimodal sleep, this routine split the night into two distinct blocks of rest separated by a quiet waking interval called "the watch".
The First Sleep: People typically retired to bed early soon after dusk (around 8:00 or 9:00 PM) and slept for roughly four hours.
The Midnight Waking: Around midnight, people naturally woke up for one to three hours, remaining calm and alert. They used this time to pray, read, talk, tend the fire, check livestock, or be intimate.
The Second Sleep: After the middle-of-the-night break, individuals returned to bed for their second sleep, which lasted until dawn. [1, 2, 3, 4, 5]
Why the Habit Changed
Preindustrial Norm: Documented widely by historian A. Roger Ekirch, this pattern was completely normal across Western civilization and preindustrial societies before the Industrial Revolution.
Rise of Artificial Light: The widespread adoption of cheap artificial lighting and structured factory schedules in the 18th and 19th centuries pushed bedtimes later and compressed human rest into a single, continuous 8-hour block (monophasic sleep).
Scientific Insight: Modern experiments show that when humans are exposed to long periods of natural darkness without artificial lighting, they naturally revert to this two-phase sleeping rhythm.
Edison:
Thomas Edison is the famous historical figure widely known for taking regular short naps at his desk and around his laboratory, despite publicly claiming he barely slept. [1, 2]
The Habit: While Edison publicly boasted about working long hours on very little nocturnal sleep, he routinely took short catnaps and power naps right at his desk or on cots placed around his workplace. [1, 2]