The Science Behind Why Humans Stopped Sleeping Twice Every Night
Before the modern era, humans commonly followed a pattern of “segmented sleep” — two distinct sleep periods during the night with a wakeful break in between. This ancient rhythm faded with industrialization and artificial lighting. Here’s how science explains this profound shift in our sleeping habits.
A Forgotten Pattern: The First and Second Sleep
Centuries ago, before electricity illuminated the night, people typically went to bed soon after sunset. They would sleep for four to five hours — known as the “first sleep” — then wake up around midnight. This period of quiet wakefulness lasted an hour or two, during which people prayed, read, talked, or even visited neighbors. Afterwards, they returned to bed for their “second sleep,” awakening refreshed at dawn. Historical records, literature, and diaries from the 16th to 18th centuries all mention this two-phase rest as entirely normal.
The Role of Darkness and Circadian Rhythm
Science provides an interesting explanation for this pattern. In the absence of artificial light, our internal body clock, or circadian rhythm, was guided by natural darkness. During long winter nights, melatonin — the hormone that regulates sleep — would rise early and stay elevated for a prolonged period. This caused the brain to induce two waves of sleep rather than one long stretch. The interval in between allowed the body to complete cycles of deep rest and dreaming more efficiently.
How Industrialization Changed Everything
With the invention of electric lighting in the 19th century, everything changed. People stayed awake later, reducing the natural length of nighttime. Factories and office jobs introduced strict schedules that demanded continuous, uninterrupted sleep to meet early morning routines. Gradually, “biphasic sleep” was replaced by the modern “monophasic” pattern — one long sleep at night.
Modern Science and the Return of the Nap
Today, researchers suggest that some people’s insomnia or nighttime wakefulness may be remnants of this ancestral rhythm. Studies show that short naps or segmented rest can improve creativity, memory, and mood echoing our ancestors’ habits.
Conclusion: A Natural Shift, Not a Flaw
The shift from two sleeps to one wasn’t an evolution of biology but a response to social and technological change. While our bodies once thrived on segmented rest, our modern world demands efficiency. Yet, science reminds us that waking at night isn’t always unnatural — it’s simply a whisper from our past.
