APA Dictionary of Psychology

a pattern in which two periods of nighttime sleep are separated by an extended period of wakefulness. It has been suggested that this pattern, called first sleep and second sleep and often involving a third period of light sleep during the day, may have been the norm for human beings before industrialization and the advent of electric lighting. See polyphasic sleep. [described by U.S. historian A. Roger Ekirch in 2001]