Infradian rhythms

Infradian rhythms: a type of biological rhythm with a frequency of less than one cycle in 24 hours, such as the menstrual cycle and seasonal affective disorder

The menstrual cycle

• Governed by monthly changes in hormone levels (oestrogen and progesterone)
• Its purpose is to regulate ovulation and prepare for pregnancy
• When pregnancy does not occur, hormones cause the womb lining to be shed
• It is approximately a 28-day cycle but can be anywhere between 24-35 days
• It may also be affected by external factors (pheromones)

Stern and McClintock (1998)

• Studied 29 women who had irregular periods
• Samples of pheromones were gathered from 9 of the women at different stages…