Splitting a full night’s rest into two distinct blocks is known as segmented or biphasic sleep. This pattern divides the sleep period into two segments separated by a period of wakefulness, departing significantly from the standard monophasic schedule most modern societies follow. Many people who struggle with middle-of-the-night wakefulness wonder if their bodies are naturally programmed for this older form of rest. Science suggests that while modern lifestyles have consolidated sleep, the biological capacity for a split schedule remains. Understanding the historical precedent and underlying biological drivers helps determine if this flexible pattern is a viable approach.
Historical Context of Segmented Sleep
The consolidated, single block of sleep is a relatively recent phenomenon, largely emerging after the Industrial Revolution. Before the widespread use of artificial lighting, especially in Western pre-industrial societies, a biphasic sleep pattern was the norm. Historian A. Roger Ekirch uncovered extensive documentation describing this common nightly routine, frequently referencing a “first sleep” and a “second sleep.”
People typically retired shortly after sunset for a “first sleep” lasting three to four hours. They would then awaken around midnight for an hour or more of wakefulness, often spent reading, reflecting, or socializing. Following this period, they would return to bed for a “second sleep” until morning. This two-part schedule was considered a normal and expected part of the nocturnal cycle, not insomnia. The shift away from this pattern occurred as artificial illumination extended the waking day and industrial work schedules emphasized consolidated rest.
The Biological Mechanism of Split Sleep
The body’s ability to accommodate segmented sleep is rooted in the “Two-Process Model” of sleep regulation. This model involves the interaction of the homeostatic drive (Process S) and the circadian rhythm (Process C). Process S is the increasing pressure to sleep that builds up the longer a person is awake, driven by the accumulation of adenosine in the brain. Process C is the internal biological clock, located in the suprachiasmatic nucleus (SCN), which dictates the timing of sleep and wakefulness over 24 hours.
A brief period of wakefulness often aligns naturally with the nadir, or lowest point, of the circadian drive for sleep, occurring around the middle of the night. At this time, homeostatic sleep pressure from the first segment has been partially relieved, and the circadian signal to stay asleep is temporarily at its weakest. This combination allows for a wakeful interval that feels restorative. This biological pattern was demonstrated in a 1990s study by Thomas Wehr, where subjects deprived of artificial light naturally reverted to a segmented sleep pattern. The wake interval often coincides with reduced melatonin levels, which facilitates a brief, calm waking period before the second sleep begins.
Practical Methods for Biphasic Sleeping
Implementing a biphasic schedule requires consistency and a commitment to total sleep duration, which should remain in the recommended seven to nine-hour range for most adults.
Siesta Method
One common modern variation is the “Siesta” method, which involves a core night sleep of five to six hours, supplemented by a longer nap of 60 to 90 minutes in the early afternoon. This longer nap is timed to allow the body to cycle through a full sleep cycle, potentially including deep sleep stages.
Historical Segmentation
Alternatively, a segmented schedule can mirror the historical pattern, consisting of two nightly sleep blocks, each lasting approximately four hours, separated by a one to three-hour wake period. This schedule typically requires an earlier bedtime to ensure both segments fit before morning obligations.
Power Nap Approach
A less intensive biphasic approach involves a six to seven-hour night sleep and a short, 20- to 30-minute power nap during the day. The short nap is designed to be restorative by remaining in the lighter stages of sleep, avoiding the grogginess associated with waking from deep sleep. Strict adherence to the planned schedule is necessary for the internal clock to adjust to the new rhythm.
Evaluating the Trade-offs of Segmented Sleep
Adopting a segmented sleep schedule presents various trade-offs. On the positive side, individuals who experience middle-of-the-night insomnia may find relief by reframing the wake period as normal rather than a sleep disorder. This reduces anxiety, making it easier to fall back asleep for the second segment. The wakeful interval also offers a quiet, productive time for focused, low-stimulus activities like reading or personal work.
The primary challenge is maintaining the schedule within a society built around a monophasic rhythm. Social engagements and rigid work hours can make it difficult to accommodate an afternoon nap or an early bedtime. A significant risk is the development of sleep debt if the total time asleep consistently falls below individual requirements. Furthermore, if a biphasic pattern involves a longer nap that is not timed correctly, waking from deep sleep can lead to sleep inertia, a temporary state of grogginess and impaired performance. Ultimately, the quality and total quantity of sleep (seven to nine hours) are more important for health than the number of segments it is divided into.