Why Some Cultures Sleep Twice a Night: The Lost Science of Biphasic Sleep

The practice of sleeping twice a night, known as biphasic or segmented sleep, was the historical norm for many cultures across the globe, especially in pre-industrial societies.

This pattern, often referred to as “first sleep” and “second sleep,” was a widespread, natural rhythm that only began to disappear with the advent of artificial lighting and the Industrial Revolution.

Here are the key reasons why some cultures slept twice a night:

1. The Natural Absence of Artificial Light

Circadian Rhythm Alignment: Before electricity and readily available gas/oil lamps, people’s schedules were heavily dictated by the sun. They would often go to bed shortly after sunset.
The Body’s Natural Segments: Research, including experiments that expose modern humans to long periods of darkness (simulating pre-industrial nights), suggests that the human body’s natural sleep-wake cycle may, in fact, be segmented. After 3-4 hours of sleep, a period of natural wakefulness often occurs, which was not viewed as insomnia but as a normal part of the night.
Melatonin and Dark Time: The long hours of darkness in the absence of light cues naturally encourage an earlier sleep onset and a mid-night awakening before the body returns to sleep until dawn.

The period of wakefulness, often lasting an hour or two between the “first sleep” and “second sleep,” was used for a variety of activities that were difficult or impractical during the day:

Quiet Chores: People would tend to the fire, check on livestock, or perform quiet, necessary household tasks.
Reflection and Spiritual Time: This quiet, peaceful time was often used for prayer, religious devotion, meditation, and contemplating dreams, which were believed to be more vivid during this “watch” period.
Social and Intimate Time: Historical records, diaries, and literature mention people getting up to chat quietly with family, visit neighbors, or engage in sexual intimacy before returning for the “second sleep.”
Uninterrupted Work: Scholars and writers often used this calm, distraction-free interval for reading, writing, or creative thinking.

3. The Shift to Monophasic Sleep

The segmented sleep pattern declined in Western cultures beginning in the 17th century and rapidly disappearing during the Industrial Revolution and the widespread introduction of artificial light (gas and electric lamps).

Artificial Light: This allowed people to push back their bedtime, staying up later into the night for work, reading, or leisure. By delaying the onset of sleep, the natural mid-night waking period was either suppressed or simply didn’t last long enough to be utilized, making it feel like an unwelcome interruption (i.e., insomnia).
Industrial Schedules: The rise of factories and standardized work hours demanded a consistent, continuous block of labor and, consequently, a single, uninterrupted block of rest to maximize daytime productivity.

Modern Biphasic Sleep (Siesta Culture)

While the “first sleep/second sleep” pattern vanished in most industrialized nations, a form of biphasic sleep still exists culturally, most famously as the siesta.

The Siesta: Common in Mediterranean, Latin American, and other warm climates (like Spain, Italy, and Greece), the siesta involves a shorter night’s sleep coupled with a long nap (often 60–90 minutes) in the early afternoon. This pattern allows people to avoid the most intense heat of the midday sun and aids in digestion and alertness for the rest of the day.

This historical context highlights that our current pattern of sleeping in one continuous block is a relatively recent, culturally-enforced habit, not a universal biological necessity.