With Polyphasic Sleep, You Can Thrive on as Little as Two Hours per Night by Philip Perry.
What we consider a “normal” sleep cycle is called monophasic. This is sleeping for one long period throughout the night. In some Southern European and Latin American countries, the style is biphasic. They sleep five to six hours per night, with a 60-90 minute siesta during midday. There is a historical precedent too. Before the advent of artificial light, most people slept in two chunks each night of four hours each, with an hour of wakefulness in-between. That’s also a biphasic system. Then there is polyphasic sleep. This is sleeping for different periods and amounts of time throughout the day.