Distribution : In polyphasic sleep, an organism sleeps at multiple times during a 24-hour cycle. Monophasic sleep occurs all at once. Given a 14-hour period of darkness in experimental conditions, humans tend towards Bimodal sleep, with two sleep periods concentrated at the beginning and at the end of the dark time. Different characteristic sleep patterns, such as the famous "early bird" and "night owl", are called chronotypes.
Genetics and sex have some influence on chronotype, but so do different habits. Chronotype is also liable to change over the course of a person's lifetime depending upon his work and exposure to light etc. Source: Wikipedia
