ultradian and infradian rhythms

• P: differences in sleep patterns of individuals are usually attributed to differences in non-biological factors. study by Tucker suggests that these differences are in large part biologically determined and may even be genetic in origin

  E: participants were studied over 11 days and nights in controlled, lab environment. researchers assessed sleep duration, time to fall asleep and amount of time in each sleep stage. they found large individual differences in each of these characteristics
• link for individual differences

  L: this means that differences between participants were not driven by circumstances, but were at least partially biologically determined
• P: the menstrual cycle is normally governed by an endogenous system - the release of hormones by the pituitary gland. however, it can also be controlled by exogenous cues

  E: when several women of childbearing age live together and do not take oral contraceptives, their menstrual cycles tend to synchronise, in one study, samples of sweat were collected from one group of women and rubbed onto the upper lips of women in the second group. groups were kept separate but their menstrual cycles became synchronised with their 'odour donor'
• link for menstrual cycles

  L: this suggests that the synchronisation of menstrual cycles can be affected by pheromones, which act in a similar way to hormones, but have an effect on the bodies of people close by rather than the body of the person producing them
• P: research by penton suggests that human mate choice varies across the menstrual cycle, an infradian rhythm, with different preferences at different stages for the cycle

  E: they found women expressed a preference for slightly feminised male faces when picking a partner. however, when in ovulatory phase of their menstrual cycle, women showed preference for more masculinised faces
• link for mate choice

  L: this study appears to demonstrate a preference for kindness and cooperation in long term mates, but a preference for males with good genes for short term liaisons so that these genes might be passed to their offspring