- Anthropologists have found evidence that sleeping in two shifts/bimodal sleeping was a norm before Pre-industrial Europe.
- Historian A. Roger Ekirch's book At Day's Close: Night in Times Past describes that households at this time started to sleep at dusk, woke up a few hours later for one or two hours, and then went to sleep again until dawn.
- During the waking period, people would relax or do some activities.
- Ekirch found references that this modality of sleeping started to disappear in the late 17th century.
- The appearance of insomnia in the literature coincides with the end of the split sleep period.
- Modern society may place unnecessary pressure on individuals to have an excellent continuous consolidated sleep every night, adding the anxiety of sleeping and perpetuating the problem.
- In the early 1990s, the psychiatrist Thomas A. Wehr conducted a laboratory experiment that showed that human sleep is biphasic in short photoperiods.
- He exposed a group of people in a short photoperiod - they were left in the darkness for 14 hours every day - for a month.
- By the fourth week, a distinct two-phase sleep emerged. They slept the first 4 hours, then woke up for 1 to 3 hours before falling into a second 4-hour sleep.
This finding suggests bi-phasic sleep is a natural process with a biological basis.
- A notable advantage of first and second sleep is that it provides two periods of increased activity, creativity, and alertness across the day, rather than one long period of activity in which sleepiness builds up across the day and the productivity wanes.
- A significant disadvantage of this kind of sleep is that today's society doesn't allow this kind of flexibility.