Key terms

• One biological rhythm is the 24-hour circadian rhythm (often known as the ‘body clock’), which is reset by levels of light. The word circadian is from the Latin ‘circa’ which means ‘about’, and ‘dian’, which means ‘day’.
• Ultradian rhythms reset more than one cycle in 24 hours and can be found in the pattern of human sleep. This cycle alternates between REM (rapid eye movement) and NREM (non- rapid movement) sleep and consists of five stages.
• Another important biological rhythm is the infradian rhythm. Infradian rhythms occur less than one cycle every 24 hours (for e.g. a woman’s menstrual cycle does not rest every morning) and can be weekly, monthly or annually.
• A monthly infradian rhythm is the female menstrual cycle, which is regulated by hormones that either promote ovulation or stimulate the uterus for fertilisation.