Biorhythms are roughly periodic changes in the metabolism or activity in living things. Well known examples are the sleep cycle, annual rhythms, or menstrual cycles. Biorhythms are usually synchronized with daily, monthly, seasonal, or annual changes in the environment. Examples of daily or circadian rhythms are the opening and closing of flowers (see nyctinasty), feeding cycles of animals during the day or night, the response of marine organisms to the tides, and, in humans, changes in body temperature and blood pressure. Monthly changes include the menstrual cycle in women. Annual rhythms include hibernation, migration, and reproductive activity. The internal mechanisms of these rhythms (sometimes referred to as the biological clock is not yet fully understood.