A third of the population has difficulty sleeping, including difficulties staying asleep throughout the night

While most sufferers find nightly awakenings disturbing, there is some evidence from our recent past that suggests this period of wakefulness happening between two discrete sleep periods was the norm.

There have been various instances of segmented sleep throughout history, ranging from medical books to court documents and diaries, and even in African and South American cultures, with a consistent reference to “first” and “second” sleep.

Charles Dickens writes in Barnaby Rudge (1840):

He knew this, even in the terror with which he began his first sleep, and threw open the window to dispel it by the presence of some thing beyond the room that had not been, as it were, the witness of his dream.

Anthropologists discovered evidence that bi-modal sleeping was the norm in preindustrial Europe. The onset of sleep was determined not by a specific bedtime, but by the availability of activities.

In his book At Day’s Close: Night in Times Past, historian A. Roger Ekirch tells how households at this period retired a few hours after dusk, awoke a few hours later for one to two hours, and then slept again until morning.

People would relax, think about their dreams, or have sex during this waking hour. Some would sew, cut wood, or read in the dark, relying on the moon or oil lamps for illumination.

Ekirch discovered that references to the first and second sleeps began to fade in the late 17th century. This is supposed to have begun in Northern Europe’s top classes and spread to the rest of Western civilization over the next 200 years.

Interestingly, the advent of sleep maintenance insomnia in the literature in the late nineteenth century correlates with the disappearance of descriptions of split sleep. As a result, modern culture may put undue pressure on individuals to get a full night’s sleep every night, contributing to sleep anxiety and perpetuating the problem.

Biological foundation

In today’s world, less dramatic versions of bi-phasic sleep can be seen, such as in cultures who take an afternoon siesta. Our body clock lends itself to such a routine, with a decrease in attentiveness in the early afternoon (the dreaded ‘post-lunch slump’).

Psychiatrist Thomas Wehr did a laboratory experiment in the early 1990s in which he exposed a group of people to a short photoperiod – that is, they were left in darkness for 14 hours every day instead of the usual 8 hours – for a month.

It took some time for their sleep to settle, but by the fourth week, they had developed a definite two-phase sleep pattern. They slept for four hours at first, then awoke for one to three hours before falling asleep again for four hours. This discovery implies that biphasic sleep is a natural process with a biological foundation.

Advantages and disadvantages

Today’s society frequently does not allow for such flexibility, thus we must comply to today’s sleep/wake routines. A continuous 7 to 9-hour unbroken sleep is typically regarded to be optimum for feeling rejuvenated. However, because we desynchronize with the external 24-hour light/dark cycle, such a timetable may not fit our circadian rhythms.

To successfully maintain a divided sleep schedule, you must get the time correct, which means starting sleep when there is a strong desire to sleep and during a low circadian point, in order to fall asleep fast and stay asleep.

One of the primary benefits of a split sleep schedule is the flexibility it provides with regard to work and family time (where this flexibility is afforded). Some people in modern culture have embraced this type of schedule because it provides two moments of heightened activity, creativity, and alertness throughout the day, as opposed to having a protracted wake period where tiredness creeps up over the day and productivity wanes.

In support of this, there is mounting evidence that naps can improve memory and learning while also raising attentiveness and mood states. Some people feel that sleep problems, such as sleep maintenance insomnia, stem from the body’s natural tendency for fragmented sleep. As a result, divided sleep regimens may be more natural for some people.

Consequences for shift work

Split sleep cycles are emerging as a viable alternative to continuous night shift work. Working at night presents the challenges of prolonged wakefulness (typically working 8 to 12-hour shifts) and circadian misalignment (working at a time of night when you would normally be asleep).

Shift workers commonly report exhaustion and decreased productivity at work, and they are more likely to develop chronic diseases such as obesity, type 2 diabetes, and heart disease.

Some companies have implemented schedules with shorter but more frequent sleep opportunities, assuming that the need for sleep will decrease as time is reduced. For example, 6 hours on/6 hours off, 4 hours on/8 hours off, and 8 hours on/8 hours off, minimizing duration on shift and shortening periods of alertness.

Split sleep/work schedules divide the day into numerous work/rest cycles, allowing employees to work many short shifts every 24 hours, separated by short off-duty intervals.

Split-shift schedules that allow for enough sleep per 24 hours may be advantageous to sleep, performance, and safety. A number of recent research have indicated that split sleep has equivalent performance benefits to one long sleep if the total sleep time per 24 hours is maintained (at around 7 to 8 hours total sleep time per 24 hours).

However, as expected, performance and safety can be compromised if wake-up and start-work timings are in the early morning. And we don’t know if these schedules provide any health benefits or lessen the risk of chronic disease.

While the difficulties of night shift work cannot be avoided, the advantage of some split shift schedules is that all workers get at least some sleep at night and are not required to remain attentive for more than 6 to 8 hours.

Although we all want to sleep well, this may not be possible for everyone’s body clock or job schedule. It could be a reversion to our pre-industrial ancestors’ bi-model sleep cycle, and it could work well in a modern industrial situation.