It is commonly believed that humans have a single sleep cycle per night. However, some research suggests that historically, humans may have experienced segmented sleep, with two distinct sleep periods at night. The concept of bifurcated sleep has roots dating back to medieval times and earlier.
Some key questions around this topic include:
What is segmented sleep?
Segmented sleep refers to a pattern of sleeping in two main periods at night. This biphasic sleep pattern typically involves an initial, longer sleep period, followed by a period of wakefulness, and then a second sleep cycle until morning.
What are the phases of human sleep?
Human sleep consists of two main types of sleep:
| Sleep Stage | Characteristics |
|---|---|
| Non-REM sleep | Consists of three phases: – N1: Transition from wakefulness to sleep – N2: Light sleep – N3: Deep, slow-wave sleep |
| REM sleep | Rapid eye movement sleep when dreaming occurs |
These sleep stages cycle throughout the night during normal monophasic sleep.
How does segmented sleep differ from monophasic sleep?
In monophasic sleep, sleep stages flow continuously through the night. In segmented sleep, there is typically a 1-2 hour period of wakefulness separating two longer sleep periods at night.
What are the proposed benefits of segmented sleep?
Some proposed benefits of biphasic sleep include:
– More vivid dreams and lucid dreaming during the second REM cycle
– Greater focus and alertness during the waking period at night
– Potential creativity boost during the middle-of-the-night awake time
History of Segmented Sleep
References to segmented sleep patterns date back centuries, suggesting this biphasic sleeping pattern was once common in human societies.
Pre-Industrial Era
Before widespread artificial lighting, many areas experienced longer winter nights and shorter summer nights. Historical records and literature from the pre-industrial era contain accounts of first and second sleep.
For instance, in medieval texts, the first sleep was referred to as “first sleep” or “dead sleep” while the second was called “second” or “morning” sleep. In between the two sleeps, people were said to have enjoyed activities like reading, praying, contemplating, or having sex.
Industrial Revolution Onward
With the Industrial Revolution and the introduction of artificial lighting, nighttime activities gradually shifted more toward single, consolidated nighttime sleep. By the early 20th century, the concept of first and second sleep had largely faded from literature and accounts of normal sleep patterns.
Electric lights allowed people to stay awake longer into the night, altering the circadian rhythms that drive sleep-wake cycles. Historical records show that before artificial lighting, average sleep durations were longer than eight hours per night. Consolidating sleep into a single block became increasingly common in industrialized regions.
Scientific Research on Segmented Sleep
While fragmented sleep may have been the norm centuries ago, some researchers in the 1990s and 2000s set out to explore whether it offers benefits in the modern world.
Early Lab Experiments
In the 1990s, psychiatrist Thomas Wehr conducted lab experiments altering participants’ exposure to artificial light. Under dim lighting conditions that mimicked pre-industrial winter days, participants began exhibiting biphasic sleep patterns. This suggested that segmented sleep may arise naturally if humans live without modern artificial lighting.
Studies on Split Sleep Schedules
Other researchers delved deeper into biphasic sleep patterns. A 2003 Australian study focused on people who slept in two segments per night by choice. The researchers found that split sleepers needed 20 minutes less total sleep time to feel refreshed.
However, the split sleepers slept only 6.5 hours total per night, lower than the recommended seven or more hours for adults. Short sleep duration may limit conclusions about cognitive performance.
It May Depend on Chronotype
More recent research found that sleep chronotype plays a role in responses to biphasic sleep schedules. In a 2013 study, participants slept four hours, were awake for two to three hours, then slept another four hours.
Morning chronotypes performed better on cognitive tests than evening chronotypes when following this protocol. The researchers proposed that split sleep may align better with innate circadian rhythms for morning persons.
Arguments Against Segmented Sleep
Despite some evidence that biphasic sleep patterns are biologically plausible for humans, there are also arguments against segmented being more beneficial than consolidated sleep:
Lack of Extensive Research
Overall, scientific research on segmented versus monophasic sleep is still quite limited. There are not yet comprehensive, large-scale studies comparing cognitive performance and other health markers. Since industrialization changed sleep habits so relatively recently, adapting to monophasic sleep may still be evolving.
Need for Sleep Consolidation
Many sleep experts emphasize the importance of sleep consolidation for proper cognitive functioning and health. Sleeping without major disruptions allows the body to fully move through the stages of sleep repeatedly through the night. Interrupting sleep with long periods of wakefulness may interfere with this process.
Unknown Long-Term Effects
The long-term physical and mental effects of routinely segmented sleep schedules have not yet been well-documented. Potential effects like fatigue, mood changes, and increased risk for conditions like obesity are not yet clear. More extensive clinical studies over time are needed.
Current Sleep Recommendations
At this time, most sleep scientists and doctors recommend consolidating sleep into a single nightly block:
| Age Group | Recommended Amount of Sleep |
|---|---|
| Newborns (0-3 months) | 14 to 17 hours |
| Infants (4-12 months) | 12 to 16 hours |
| Toddlers (1-2 years) | 11 to 14 hours |
| Preschoolers (3-5 years) | 10 to 13 hours |
| School-age (6-12 years) | 9 to 12 hours |
| Teens (13-18 years) | 8 to 10 hours |
| Adults (18-60 years) | 7 or more hours |
| Older Adults (61-64 years) | 7 to 9 hours |
| Older Adults (65+ years) | 7 to 8 hours |
Splitting sleep into two segments is not currently recommended for most adults without extenuating circumstances. However, some flexibility is reasonable if segmented sleep occurs naturally for certain individuals.
Conclusion
In summary, biphasic sleep with two distinct sleep periods per night was common historically. The Industrial Revolution and widespread artificial lighting likely precipitated a shift toward today’s consolidated monophasic sleep pattern.
Some research suggests potential cognitive and health benefits from segmented sleep for subsets of the population. However, large-scale studies of long-term effects are still scarce. At this time, one continuous block remains the recommended sleep pattern for adults, while being open to individual variability. More rigorous studies comparing biphasic and monophasic sleep would further illuminate this evolving area of research.
