infradian rhythms

• infradian rhythms last longer than 24 hours, for e.g. the menstrual cycle.
• seasonal affective disorder (SAD) is when one feels depressed in the winter and normal in the summer. this may be because it becomes dark in the winter more often. the lack of light produces the production of melatonin and less serotonin which can cause depression. which is why this may happen.
• another explanation of SAD is that in winter we disrupt our circadian rhythm since we go to sleep earlier due to less light exposure. thus putting our rhythms out of phase and impacting our biological function.
• eastman: randomly assigned 96 patients w/ SAD to one of three bright-light treatments that are 10 to 20 more times brighter than normal indoor lights. they were divided into 3 groups:

  1. 1hr 30 of bright light in the morning
  2. 1hr 30 of bright light in the night
  3. (control group) morning placebo of two deactivated negative-ion generators.
• findings: after 3 weeks, significantly more people in group 1 show complete or near-complete remission of their symptoms. response to evening light was also better but not at a level of significance. importantly, eastman's group found that effective phototherapy fostered full remission of depression.
• in the menstrual cycle the endogenous pacemakers are the hormones FSH and LH which are released by the pituitary gland that ripens the egg and triggers the release of oestrogen and progesterone, preparing the womb. the exogenous zeitegebers would be women who live together, thus having their period synced. (pheromones)
• russel (1980): female menstrual cycles become synchronised with other females due to odour exposure. he did this by rubbing sweat samples of one group of women to the upper lip of another group's. although they were separate, their periods were synchronised. suggesting that external factors should be considered as there is evidence of them having an effect.