Biphasic

A biphasic system is one which has two phases. For example in chemistry, when oil and water are poured into a beaker, they form a biphasic system. In chronobiology, biphasic sleep refers to a nap or siesta in addition to the usual sleep episode at night. Monophasic sleep means just one sleep episode per day.

In psychiatry it may refer to a sigmoidal curve that goes downward as the axis increases because a competing behavior is impeding. This competing behavior is called stereotypy. Stereotypy is when individuals or animals do simple tasks over and over (ex: experimental rat making circles for several hours).