There is a fascinating article on sleep in The New York Times online. It seems that ‘the seamless night’ of uninterrupted sleep is an anomaly – a product of modern society:
...for many centuries, and perhaps back to Homer, Western society slept in two shifts. People went to sleep, got up in the middle of the night for an hour or so, and then went to sleep again. Thus night — divided into a “first sleep” and “second sleep” — also included a curious intermission. “There was an extraordinary level of activity,” Ekirch told me. People got up and tended to their animals or did housekeeping. Others had sex or just lay in bed thinking, smoking a pipe, or gossiping with bedfellows. Benjamin Franklin took “cold-air baths,” reading naked in a chair.
Our conception of sleep as an unbroken block is so innate that it can seem inconceivable that people only two centuries ago should have experienced it so differently. Yet in an experiment at the National Institutes of Health a decade ago, men kept on a schedule of 10 hours of light and 14 hours of darkness — mimicking the duration of day and night during winter — fell into the same, segmented pattern. They began sleeping in two distinct, roughly four-hour stretches, with one to three hours of somnolence — just calmly lying there — in between. Some sleep disorders, namely waking up in the middle of the night and not being able to fall asleep again, “may simply be this traditional pattern, this normal pattern, reasserting itself,” Ekirch told me. “It’s the seamless sleep that we aspire to that’s the anomaly, the creation of the modern world.”