Each person has its biological rhythm.
There are people who sleep at night and are more active during the day and there are those who are night birds because they feel more awake at night. All this depends of the person''s habits and of the kind of work they perform. This means that if a person works during the day, it will feel more tired at night; on the other hand, if a person works nights it will need to rest during the day.
Furthermore, if a person consums a lot of cafeine, its sleep will not be so relaxing as some one who does not. If one drinks coffee in the evening, insomnia will arise or even a bad night''s sleep. This is the general assumption becaise there are people whose sleep is not affected by cafeine.
Another biological rhythm that exists only in women is menstruation that goes through a cycle during which several changes occur and that are different from woman to woman.
More summaries about the Biological Rhythms