Infradian and ultradian rhythms

• The human menstrual cycle is about 28 days, it is an infradian rhythm.

  Rising levels of estrogen cause the ovary to release an egg. Then progesterone helps the womb lining to thicken, readying the body for pregnancy. If pregnancy does not occur, the egg is absorbed into the body and the womb lining comes away and leaves the body.
• Stern and McClintock studied 29 women with irregular periods. Pheromones were taken from some at different stages of their cycles, via a cotton pad under their armpits. These pads were cleaned with alcohol and later rubbed on 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 (SAD) is a depressive disorder with a seasonal pattern - another infradian rhythm.

  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 rhythm on circannual cycle.
• SAD may be caused by the hormone melatonin. During the night, the pineal gland secretes melatonin until dawn when there is an increase in light.

  During winter, the lack of light in the morning means secretion goes on for longer.

  This has a knock-on effect on the production of serotonin in the brain.
• The stages of sleep are an ultradian rhythm. Sleep pattern occurs in 90-minute periods (i.e. more than one cycle in 24 hours - ultra diem meaning 'beyond' a day).

  Divided into five stages, each characterised by a different level of brainwave activity (using an EEG).
• Five stages of sleep have been identified.

  Stages 1 and 2: light sleep. Person may be easily woken. Brainwaves become slower and more rhythmic (alpha waves), slowing further as sleep becomes deeper (beta waves).

  Stages 3 and 4: difficult to rouse someone. Deep sleep or slow wave sleep characterised by delta waves which are slower still and have a greater amplitude.

  Stage 5: REM (rapid eye movement) sleep. Fast, jerky activity of the eyes. Body paralysed yet brain activity speeds up.
• One limitation of Stern and McClintock's study is the methodology used in synchronisation studies. Commentators argue that there are many factors that may change a woman's menstrual cycle and act as confounding variables in research (e.g. stress, changes in diet). So any pattern of synchronisation is what we would expect to occur by chance. Also, research involves small samples of women and relies on them self-reporting onset of their own cycle (may be inaccurate). This suggests that important aspects of synchronisation studies may lack validity.
• One limitation of research in pheromones is the use of animal studies. The role of pheromones in animal sexual behaviour is well-documented and is the basis for most of our knowledge. For example, sea urchins release pheromones into the surrounding water so other urchins eject their sex cells simultaneously. In contrast, evidence for the effects of pheromones on human behaviour remains speculative and inconclusive.
• One strength of pheromone studies is research on the menstrual cycle shows its evolutionary value.

  It may have been advantageous for our ancestors to menstruate together and become pregnant around the same time, so offspring could be cared for collectively, increasing their chances of survival. However, Schank questions the validity of this perspective - too many females syncing within a group would produce competition for better males. Therefore, the avoidance of synchrony would appear to be the most adaptive evolutionary strategy and therefore the one that would be naturally selected.
• Another strength of research into biological rhythms is evidence supports qualitatively different stages in sleep. A study by Dement and Kleitman monitored the sleep patterns of nine participants in a sleep lab and found evidence for stages of sleep, specifically REM. The REM activity during sleep correlated with dreaming; brain activity varied according to how vivid the dreams were; participants woken during dreaming reported accurate recall of their dreams. The study suggests that REM sleep is a distinct ultradian rhythm and an important part of the ultradian sleep cycle.