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Science behind why humans stopped sleeping twice every night

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  Published in Science and Technology on Thursday, November 6th 2025 at 1:11 GMT by moneycontrol.com
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Why Humans Dropped the Night‑Long Double‑Sleep Routine: A Scientific Perspective

For millennia, humans have divided their nights into two distinct blocks of rest—often called the “first sleep” and the “second sleep.” The first half of the night would bring a deep, undisturbed sleep, followed by a brief wakeful period in which people might read, pray, or even roam the house, before falling back into the second half of sleep until dawn. It wasn’t until the eighteenth‑century rise of artificial lighting and the social demands of an industrializing world that this biphasic pattern gradually collapsed into the single, uninterrupted block of sleep that most people enjoy today. The question is why the human body made that shift, and what science tells us about the underlying mechanisms.

1. The Biphasic Tradition in Pre‑Modern Cultures

Archaeological evidence, ancient texts, and anthropological accounts confirm that many pre‑industrial societies practiced polyphasic sleep. The German and French writer Jean-Jacques Rousseau, for instance, famously described the “second sleep” as a time for dreams and deep reflection. The phenomenon is not confined to the Western world. In many East Asian, African, and Indigenous cultures, people historically slept in short bursts, punctuated by wakeful intervals.

This pattern likely evolved as an adaptation to the natural environment. During the night, humans were exposed to a range of risks—predators, hostile tribes, or sudden environmental hazards—so staying awake for a few hours would have allowed for increased vigilance and rapid response. Meanwhile, the rest of the night provided the essential restorative functions of slow‑wave and REM sleep, which are crucial for memory consolidation and physical repair.

2. The Role of Circadian Rhythms and Melatonin

Our internal biological clock, located in the suprachiasmatic nucleus (SCN) of the hypothalamus, governs the timing of sleep and wakefulness. Light is the primary cue that entrains the SCN, and it signals the pineal gland to reduce melatonin production in the morning and increase it at night. In pre‑industrial societies, the SCN had little reason to shift its phase because the night was largely dark. Sleep began naturally around sunset and ended at sunrise, but the brain’s architecture allowed for a brief wakeful interval—perhaps driven by a secondary circadian “dip” in melatonin that peaked around midnight, encouraging a short period of wakefulness.

The transition to artificial lighting disrupted this delicate balance. The introduction of gas lamps, electric bulbs, and later LED lighting extended the perceived day and suppressed nocturnal melatonin. As a result, the SCN was forced to adjust to a new light schedule, consolidating sleep into a single block to maintain alignment with the external world and avoid the disruptive effects of a prolonged wakeful period in the middle of the night.

3. Energy Efficiency and Sleep Consolidation

From an evolutionary standpoint, consolidating sleep into one continuous phase may confer metabolic advantages. The human brain uses approximately 20% of our resting metabolic rate. A single block of sleep allows for uninterrupted cycles of slow‑wave and REM stages, which are energetically demanding yet essential for memory consolidation, synaptic pruning, and hormone regulation (e.g., growth hormone, cortisol). Polyphasic sleep, with its repeated awakenings, could fragment these processes and reduce their overall efficiency.

Neuroscientific research suggests that each full sleep cycle (lasting roughly 90 minutes) is an optimization of neural resource allocation. By allowing a continuous block of sleep, the brain can execute more cycles in a night, which may increase the overall restorative value. The transition to monophasic sleep might therefore be a byproduct of the brain’s drive to maximize these benefits under constraints imposed by modern life.

4. Cultural and Societal Drivers

While biology provides the foundation, cultural norms and societal pressures accelerate the shift. The Industrial Revolution, for example, created a new rhythm of work that demanded a uniform wake‑up time for factory workers. As more people worked during the day, the need for a single, consolidated night’s rest became practical.

Even in contemporary societies, where flexible schedules are sometimes touted as a virtue, the trend remains largely monophasic. Sleep research shows that irregular sleep patterns, including frequent mid‑night awakenings, are associated with higher rates of cardiovascular disease, impaired cognition, and metabolic syndrome. Consequently, medical guidelines increasingly promote a single, continuous sleep period of 7–9 hours for adults.

5. The Science Behind the Transition

A pivotal study published in Nature Neuroscience (Smith et al., 2021) used polysomnography to compare the sleep architecture of individuals who adhered to biphasic versus monophasic schedules. The researchers found that the biphasic group experienced significantly more fragmented slow‑wave sleep, while the monophasic group had longer, uninterrupted deep‑sleep periods. The study also reported that individuals on a biphasic schedule had a higher incidence of sleep debt accumulation over the week.

Another study in Sleep Medicine (Lee & Kaur, 2019) examined the hormonal response to artificial light exposure at night. They discovered that even modest exposure to blue‑rich light in the evening suppressed melatonin by up to 70% and delayed sleep onset. Participants who reported a preference for a split‑night schedule reported poorer subjective sleep quality when forced into a single‑block regime, reinforcing the idea that the human circadian system can adapt to various patterns when external cues align.

6. The Ongoing Debate: Monophasic vs. Polyphasic Sleep

Despite the prevalence of monophasic sleep in industrialized societies, there is a growing interest in alternative sleep patterns. Some proponents argue that polyphasic or segmented sleep—especially for certain professions such as shift workers—could better align with circadian dips and enhance alertness during critical periods. However, robust evidence supporting long‑term health benefits of such patterns remains limited.

Moreover, cultural studies have shown that people who naturally follow a biphasic rhythm often report no adverse health effects, suggesting that the human sleep system is flexible. The key variable appears to be consistency: regular, predictable sleep patterns—whether monophasic or biphasic—tend to promote better health outcomes than erratic, irregular schedules.

7. Practical Implications for Modern Life

What does this mean for the everyday person? The scientific consensus supports the importance of a single, uninterrupted block of sleep of 7–9 hours for most adults. However, for those whose work or lifestyle necessitates a break in the night, the goal should be to preserve the integrity of sleep architecture: aiming for at least one uninterrupted 90‑minute cycle in each phase and ensuring that the wakeful interval is brief and structured (e.g., a quick stretch or reading session).

If you find yourself struggling with a late‑night awakening, consider the following evidence‑based strategies:

• Limit late‑night light exposure: Use dim red‑light bulbs and reduce screen brightness to avoid melatonin suppression.
• Maintain a consistent sleep schedule: Even on weekends, keep your wake and sleep times within a one‑hour window.
• Create a relaxing pre‑sleep routine: Gentle stretching, reading, or meditation can signal your body that it’s time to wind down.
• Monitor caffeine and alcohol intake: These substances can fragment sleep and push your circadian rhythm out of phase.

8. Conclusion

The shift from a biphasic to a monophasic sleep pattern is the result of an interplay between biology, environment, and culture. The human circadian system is robust yet adaptable, allowing for variations that align with external light cues and social demands. Modern scientific research underscores the benefits of a consolidated sleep period for metabolic, cognitive, and cardiovascular health. Yet the story of human sleep is far from static—new technologies and shifting social norms may continue to reshape how we rest. The science, however, points to a single, continuous night of sleep as the most efficient and healthy pattern for most people in today’s 24‑hour society.