infradian rhythms

• Infradian Rhythms are biological cycles lasting more than 24 hours and are greater than circadian rhythms. Infradian rhythms can last days, weeks, months or even be annually once per year.
• One example is the menstrual cycle in women, which is regulated by hormone secretions, namely oestrogen and progesterone secreted by the ovaries and occurs monthly with the average timing being 28 days although this can vary. Originally it was thought to be controlled by the hypothalamus acting as an endogenous pacemaker, however evidence has shown that exogenous zeitgebers play a part too.
• Some people suffer from a depressive condition called Seasonal Affective Disorder (SAD) during the winter months and recover during the summer. SAD sufferers experience severe symptoms from seasonal changes such as lowering of moods and depression, which is thought to be brought about due to less light and increased melatonin production in darkness by the pineal gland. Serotonin is converted into melatonin and low amounts of serotonin (since it is being converted to melatonin) has been linked with chronic depression.