Before the industrial revolution people actually didn't sleep in one 8 hour block. Historical research has shown that people actually slept over a 10 hour period but in two phases. They would go to bed much earlier and sleep in a 4 hour block and then by up for a couple hours before going into 'second sleep'.
In a study by Thomas Wehr, he had volunteers remain in total darkness for 14 hours everyday. No lamps or any other light was allowed. This was to simulate what it was like for people before artificial light. Eventually the subjects changed their sleeping pattern to two 4 hour blocks of sleep.
To do this kind of sleep, start to shut off all screens in the early evening and dim the lights. If you need to pass the time read books or talk to your family. This should wind down your body clock and ease you into sleep. It's also recommended that you sleep with the blinds open to wake up with sunlight. A sunrise alarm clock is a good alternative if you absolutely need to be up by a certain time.