infradian rhythms

• • take longer than a day to complete
  • SAD: seasonal affective disorder is a circannual rhythm (yearly)
  • referred to as winter blues as symptoms are triggered during winter months due to shorter daylight hours
  • during the night, the pineal gland secretes melatonin until dawn when there is light
  • during winter months the lack of light means the secretion process happens for longer which can affect the production of serotonin in the brain (lowered mood)
  • EZ (light) entrains our body to produce serotonin later and for less time because of the amount of light in the environment
• • most effective treatment for SAD is light/phototherapy
  • a box which stimulates very strong light to reset the internal body clock and promotes production of serotonin by stimulating the SCN which has an accumulative effect to build up serotonin
  • phototherapy reduces effects of SAD in about 80% of people
  • favoured in economy/over SSRIs because they are safer
  • shows that light is a factor, validates EZ entraining EP
  • however can produce headaches and eye strain
• • however, infradian rhythms must only be a partial explanation for depression otherwise we would expect everyone to have SAD during the winter months
  • some people may be more susceptible to SAD due to genetics
  • suggests that distal cause is our underlying biology
  • phototherapy has 80% efficacy, so for 20% of people it doesn't work which suggests there is another explanation and infradian rhythms are not the full explanation
• • Exogenous Zeitgebers don't have the same effect in all environments, e.g. people who live in the arctic circle or places where there is very little darkness in summer and no light in winter shows that EZ are not always important
  • people in arctic circle are said to have similar sleep patterns all year round, despite spending 6 months in total darkness
  • suggests that sleep/wake cycle is primarily controlled by EP that can override environmental changes in light
  • decreases the validity of EZ