The BBC reports that the popular belief of eight hours of sleep being optimal is a development of the 19th century, and that people have had a very different pattern of sleep before. In previous times humans usually had a first period of sleep of several hours, followed by a night-time awake phase, which in turn is followed by a second period of sleep of several hours. Many historical sources reference this bi-modal sleep pattern as if it was common knowledge, which appears to have been lost in the course of the Industrial Revolution (particularly with the advent of street lighting). Between the two periods of sleep people would be awake, and they meditated their dreams, prayed, had sex with their partner, or even visited their neighbours. It appears that many sleep-related problems may stem from the fact that the eight hour sleep is unnatural.