A01

• An infradian rhythm lasts more than 24 hours, such as the menstrual cycle which is about 28 days. During this time, increased levels of oestrogen cause the ovary to be developed and released followed by a rise in the hormone progesterone which helps the womb lining to grow thicker. 
• MicClintock aimed to investigate to what extent the menstrual cycle was influenced by exogenous factors, such as pheromones. 29 women with irregular periods were studied. Nine women had a sample of their pheromones taken via a cotton pad in their armpit, which were rubbed on the upper lips of the other participants. Results revealed that around 70% of the women experienced changes to their cycles, bringing them closer to the cycle of their ‘odour donor’.
• An ultradian rhythm is anything less than 24 hours, such as the sleep cycle, which goes through 5 distinct stages and spans roughly 90 minutes. A person will go through several of these cycles throughout the night. Each stage is characterised by a different level of brainwave activity (as shown on EEGs) and stage 5 is where rapid eye movement (REM) occurs and dreams happen.