AO1

• Infradian rhythms take more than 24 hours to complete.
• Examples of infradian rhythms are the menstural cycle and seasonal affective disorder (SAD).
• SAD is a form of depression that has a seasonal onset. It is characteristed by a persistent low mood alongside a general lack of activity and interest in life and symptoms are triggered in the winter months when the number of daylight hours is shortened.
• The hormone melatonin has been implicated in causing SAD. During the night, melatonin is released until dawn when light increases. In the winter months the lack of light in the morning does not stop the prouction of melatonin which seems to have a knock on effect on the product of serotonin, which had been linked to the onset of depressive symptoms.
• Ultradian rhythms take less that 24 hours a day to complete.
• An example of an ultradian rhythm is the sleep cycle.
• Sleep cycle

  Stage 1 and 2 - light sleep, easily woken, brainwave patterns slow (alpha waves) and become even slower (theta waves)
• Sleep cycle

  Stages 3 and 4 - delta waves, deep sleep and is difficult to wake someone from.
• Sleep cycle

  Stage 5 (REM) - body is paralysed yet brain activity speeds up and it resembles the awake brain, research suggests the REM activity is highly correlated with the experience of dreaming.
• Reinberg found that a female who spent 3 months in a cave with only the light of a small lamp, shortened her menstural cycle to 25.7 days and it took a year to readjust back to a 'normal' cycle. This shows that infradian biological rhythms can be influenced by exogenous zeitgebers such as light.
• Further research has found that menstural cycles can become synchronised by exposing women to other women's pheromones they secrete whilst on their period.