Infradian rhythms, where the cycle is longer than a day, include the menstrual cycle (28 days), testosterone secretion in males (21 days) and seasonal mating amongst animals.
The most researched of these is the menstrual cycle. This relates to activity in the endocrine system that prepares the womb for the possibility of conception after egg cells are released. Several hormones are involved, coordinated by the pituitary gland. This gland may be influenced by levels of light by levels of light and the secretion of melatonin.
This view is supported by the research of Reinberg (1967)
Aim: To investigate the influence of light levels on biological rhythms.
Procedure: A young woman spent 3 months in a cave, with no external source of light, the only light available being a miner’s lamp. The effects on the sleep/waking cycle and the infradian cycle of menstruation were noted.
Results: As in the Siffre study the woman’s day lengthened, in this case to 24.6 hours. Her menstrual cycle shortened to 25.7 days. At the end of the study, it took almost a year for her menstrual cycle to return to normal.
Conclusion: The lack of light as a Zeitgeber resulted in…