Infradian rhythms - AO1

• Infradian rhythms are those that take longer than 24 hours to complete. Examples of these are the menstrual cycle and seasonal affective disorder (SAD).
• The menstrual cycle is governed by hormones roughly on a 28 day basis
• Oestrogen levels rise to release an egg from the ovary and progesterone helps the womb lining to grow. If there is no pregnancy then the womb lining sheds.
• Stern and McClintock (1998) found that women could synchronise their periods by being exposed to other womens’ pheromones.
• SAD is a type of depressive disorder which is seasonal. When there are reduced daylight hours in the winter it can trigger a low mood in sufferers.
• This is because at night our pineal glad detects the dark and produces a hormone called melatonin. In the winter it is darker first thing in the morning and so this tricks the brain in producing melatonin for longer. This can affect the production of serotonin, which is a neurotransmitter linked to the cause of depressive symptoms.
• SAD is called a circannual rhythm as it can also be considered a circadian rhythm due to the disruption of the sleep-wake cycle.