Segmented sleep, also known as biphasic or polyphasic sleep, is the practice of dividing rest into two or more distinct periods separated by an interval of wakefulness. The most common form is biphasic sleep, involving two main sleep segments over a 24-hour cycle. This contrasts with the modern standard of monophasic sleep, which is a single consolidated block of rest, typically seven to nine hours long. This article evaluates the historical basis, biological mechanisms, and current scientific evidence regarding whether this segmented pattern is a healthy alternative for modern humans.
Historical Context of Segmented Sleep
This pattern was the dominant practice in Western societies before the widespread adoption of artificial lighting. Historical records, including diaries and literature from the 15th through the 18th centuries, frequently describe a period of “first sleep” and “second sleep.” People typically retired shortly after sunset for about four hours. This initial sleep was followed by an hour or two of wakefulness around midnight, often used for quiet activities like reading or socializing. They would then return to bed for their “second sleep,” lasting until dawn. This suggests that the human body’s natural tendency, when not constrained by artificial light, may be less consolidated than the contemporary eight-hour block.
The Biological Basis of Biphasic Sleep
The body’s sleep-wake cycle is regulated by two primary biological forces: the circadian rhythm and the homeostatic sleep drive. The circadian rhythm dictates a natural 24-hour cycle of alertness and sleepiness. The homeostatic sleep drive, often called “sleep pressure,” builds up throughout the day the longer a person remains awake. In a natural light-dark environment, this interplay creates a window where a middle-of-the-night waking is biologically plausible. Melatonin, the hormone that promotes sleep, begins to rise at dusk. Sleep pressure usually dissipates significantly after the first four-hour sleep segment. This temporary dip allows for an alert period between sleep segments before the circadian drive reasserts itself for the second phase.
Evaluating the Health Outcomes
Segmented sleep is not detrimental, provided the total sleep time remains within the recommended range of seven to nine hours. Research suggests that if total sleep is restricted, splitting it into a nocturnal block and a nap can lead to better cognitive performance than a single continuous block. However, when total sleep opportunity is adequate (around eight hours), performance measures like vigilance, working memory, and mood are comparable between segmented and continuous schedules.
The primary concern with forcing a segmented schedule is the potential for sleep fragmentation, which negatively affects sleep architecture. Fragmented sleep disrupts the necessary progression through non-rapid eye movement (NREM) and rapid eye movement (REM) stages, required for memory consolidation and emotional processing. The overall health outcome depends more on maintaining sufficient total sleep time and quality than on the number of segments.
Practical Considerations for Modern Life
For those considering a segmented schedule, a biphasic pattern incorporating a short midday nap is often a safer and more manageable option than the historical two-part night sleep. A nap of 20 to 90 minutes in the early afternoon aligns with the natural circadian dip and can boost cognitive function and mood without causing sleep inertia. Consistency is vital; both the nighttime sleep and the daytime nap must occur at roughly the same time every day to keep the circadian rhythm synchronized.
During middle-of-the-night wakefulness, it is important to minimize light exposure, especially blue light from screens, to prevent suppressing melatonin production. Segmented sleep is not recommended for individuals with underlying sleep disorders like insomnia, where an inability to stay asleep is already a problem, or for shift workers whose schedules already complicate circadian alignment. Warning signs that the pattern is not working include chronic daytime sleepiness, increased irritability, or a persistent struggle to focus.