Biphasic sleep refers to a sleep pattern in which you have two separate periods of sleep within a 24 hour period. This could be from having a typical block of sleep during the night, and then topping this up with a nap during the day. There are good arguments for napping during the day. Not only does it seem that our bodies actually really want this – we have a massive drop in alertness and awakeness in the afternoon, sometime between 1pm and 3pm, readying us for sleep – but there is a huge amount of scientific research showing that napping is really good for us. Even small amounts of sleep are beneficial.
There have also been suggestions that before the industrial revolution humans had a kind of biphasic sleep whereby a night’s sleep was split into two distinct periods, with a period of wakefulness in the middle of the night, which was spent talking, having sex, writing letters, or maybe even visiting a neighbour.