For much of European history it was common to sleep in two chunks of about four hours each, separated by a one- to two-hour period of waking activity:
[Historian Roger] Ekirch found that references to the first and second sleep started to disappear during the late 17th Century. This started among the urban upper classes in northern Europe and over the course of the next 200 years filtered down to the rest of Western society.
By the 1920s the idea of a first and second sleep had receded entirely from our social consciousness.
He attributes the initial shift to improvements in street lighting, domestic lighting and a surge in coffee houses - which were sometimes open all night. As the night became a place for legitimate activity and as that activity increased, the length of time people could dedicate to rest dwindled.
In some Asian and Latin cultures an after-lunch siesta is common, leaving less of a need for a full eight hours at night. David Randall suggests rethinking sleep, drawing on examples of flexible schedules from Google and Major League Baseball:
Rather than helping us to get more rest, the tyranny of the eight-hour block reinforces a narrow conception of sleep and how we should approach it. Some of the time we spend tossing and turning may even result from misconceptions about sleep and our bodily needs: in fact neither our bodies nor our brains are built for the roughly one-third of our lives that we spend in bed....
Several Major League Baseball teams have adapted to the demands of a long season by changing their sleep patterns. Fernando Montes, the former strength and conditioning coach for the Texas Rangers, counseled his players to fall asleep with the curtains in their hotel rooms open so that they would naturally wake up at sunrise no matter what time zone they were in — even if it meant cutting into an eight-hour sleeping block. Once they arrived at the ballpark, Montes would set up a quiet area where they could sleep before the game. Players said that, thanks to this schedule, they felt great both physically and mentally over the long haul.
Social norms spread in interesting and surprising ways. This can often make us better off as individuals and a society. But when it invades one's private life, rest, and even your dreams it has gone too far. Let your body be the one to tell you when to sleep.
See also: Sleep deficits may be associated with ADHD.