Biphasic sleep (or bimodal or bifurcated sleep) is the practice of sleeping during two periods over 24 hours, while polyphasic sleep refers to sleeping multiple times - usually more than two. Each of these is in contrast to monophasic sleep which is one period of sleep over 24 hours. Segmented sleep and divided sleep may refer to polyphasic or biphasic sleep, but may also refer to interrupted sleep where the sleep has one or several shorter periods of wakefulness. A common form of biphasic or polyphasic sleep includes a nap, which is a short period of sleep, typically taken between the hours of 9am and 9pm as an adjunct to the usual nocturnal sleep period.