Sleeping eight hours straight through may be abnormal:
In 2001, historian Roger Ekirch of Virginia Tech published a seminal paper, drawn from 16 years of research, revealing a wealth of historical evidence that humans used to sleep in two distinct chunks.
His book At Day's Close: Night in Times Past, published four years later, unearths more than 500 references to a segmented sleeping pattern - in diaries, court records, medical books and literature, from Homer's Odyssey to an anthropological account of modern tribes in Nigeria.
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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.
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Today, most people seem to have adapted quite well to the eight-hour sleep, but Ekirch believes many sleeping problems may have roots in the human body's natural preference for segmented sleep as well as the ubiquity of artificial light.
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The idea that we must sleep in a consolidated block could be damaging, he says, if it makes people who wake up at night anxious, as this anxiety can itself prohibit sleeps and is likely to seep into waking life too.
I've had lots of nights in my life in which I'd sleep for a chunk, then wake up for an hour or two and sleep another chunk. Probably most people have, and we'd be better off accepting them as our bodies' attemps to reset to a natural sleep pattern instead of dreading them as bouts of insomnia. On the nights when I just accept the awake-in-the-middle session and do something like read or watch an hour of late-night television, I seem to get much more rest than on the nights when I fidget and fight it and try to get back to sleep in a hurry.
I seem to have more of the two-session sleeps when I'm on vacation. I think having the next day's job on our minds probably discourages the three-stage sleep, since it takes longer. Work has forced us into an 8-8-8 pattern that may be unnatural but that helps keep things simple. We like simple.