ultradian & infradian rhythms

• key term - infradian rhythm

  a type of biological rhythm with a frequency of less than one cycle in 24 hours such as menstruation or seasonal affective disorder.
• key term ultradian rhythm

  a type of biological rhythm with a frequency of more than one cycle within 24 hours such as the stages of sleep (the sleep cycle)
• Menstrual cycle

  Infradian rhythm governed by monthly changes in hormone levels which regulate ovulation
• Menstrual cycle


  1. Time between first day of a woman's period when the womb lining is shed to the day before her next period

  2. Rising levels of oestrogen cause the ovary to develop & release an egg (ovulation)

  3. Progesterone helps the womb lining thicken, readying body for pregnancy

  4. If pregnancy doesn't occur, egg is absorbed into the body & womb lining comes away - leaves the body (menstrual flow)
• Typical female menstrual cycle

  • About 28 days
• synchronising the menstrual cycle

  Although the menstrual cycle is an endogenous system, evidence suggests it may be influenced by exogenous factors e.g., cycles of other women.

  A study by McClintok and Stern in 1998 demonstrated how menstrual cycles may synchronise as a result of the influence of pheromones.
• synchronising the menstrual cycle - procedure

  They studies 29 women with histories of irregular periods. Pheromones were taken from some at different stages of their cycles, via a cotton pad placed under their armpits for at least 8 hours to ensure the pheromones were picked up. These pads were cleaned with alcohol and frozen and later rubbed onto the upper lips of the other participants.

  68% of women experienced changes to their cycle which brought them closer to the cycle of their ‘odour donor’.
• seasonal affective disorder - infradian rhythm

  SAD is a depressive disorder (low mood, lack of activity) with a seasonal pattern.

  SAD is often called the ‘winter blues’ because the symptoms are triggered during the winter months when the number of daylight hours becomes shorter.

  SAD is a particular type of infradian rhythm called a circannual rhythm as it is subject to a yearly cycle.

  However, it can be classed as a circadian rhythm because SAD may be due to the disruption of the sleep wake cycle and this can be due to prolonged periods of daily darkness during winter.
• seasonal affective disorder and melatonin

  Psychologists have hypothesised that the hormone melatonin is implicated in the cause of SAD. During the night the pineal gland secretes melatonin until dawn where there is an increase in light. During winter – lack of light in the morning à secretion carries on longer. This is thought to have a knock on effect on the production of serotonin in the brain – a chemical that has been linked to the onset of depressive symptoms.
• ultradian rhythms

  Stages of sleep are an ultradian rhythm. Psychologists have identified 5 stages of sleep that all together span approximately 90 minutes – cycle that continues throughout the night. Each stage is characterised by a different level of brainwave activity which can be monitored using an EEG.
• stage of sleep - 1 & 2 - ultradian rhythms

  Stages 1 & 2 – light sleep where a person might easily be woken. In stage 1, brain waves are at a high frequency and have a short amplitude. These are alpha waves, the alpha waves continue but there are occasional random changes in pattern called sleep spindles.
• stages of sleep - 3 & 4

  Stages 3 &4 – this is known as deep sleep or slow wave sleep (SWS). The brain waves are delta waves, with lower frequency and higher amplitude. It is difficult to wake someone at this point.
• stages of sleep - 5 & 6

  Stages 5 & 6 (REM sleep) – body is paralysed yet brain activity speeds up (closely resembles that of an awake brain). During this time the brain produces theta waves and the eyes occasionally move around thus rapid eye movement. Dreams are most often experienced during REM sleep but may also occur in deep sleep.