Infradian and Ultradian Rhythms

• A type of biological rhythm which has a duration of over 24 hours is called an infradian rhythm
• Infradian rhythms include the female menstrual cycle in humans and seasonal affective disorder
• The menstrual cycle is governed by monthly changes in hormone levels which regulate ovulation
• Endogenous factors affecting menstruation:

  1. Oestrogen- increases to cause the ovaries to develop an egg and release it
  2. Progesterone- thickens womb lining, prepares uterus for pregnancy
• Exogenous factors affecting menstruation:

  1. health
  2. age
  3. smoking/drinking
  4. physical activity
• Stern and McClintock (1998) found that the menstrual cycle can be affected by exogenous factors such as the cycle of other women (synchronisation from pheremones)
• The aim of Stern and McClintock's study was to investigate whether exogenous factors can affect the menstrual cycle
• Findings of Stern and McClintock's study:

  • 68% of women experienced changes to their cycle and were brought closer to the cycle of their 'odour donor'
• Conclusion of Stern and McClintock's study:

  • Pheremone levels impact the regularity of the menstrual cycle
  • Exogenous zeitgebers can lead to the synchronisation of menstrual cycles
• Seasonal affective disorder (SAD) is a form of depression triggered by the winter months due to reduced daylight hours
• Symptoms of SAD include persistent low mood, low interest in activities and life
• Psychologists have hypothesised the hormone melatonin is involved in the onset of SAD
• Melatonin is secreted by the pineal gland until dawn when light levels increase
• During winter, the lack of light means that melatonin is secreted for longer
• Low melatonin levels impact the secretion of serotonin in the brain
• Ultradian rhythms are biological rhythms that occur on a timescale of less than 24 hours
• An example of an ultradian rhythm is the sleep cycle
• There are 5 identified stages of sleep
• Most of what we know about the sleep cycle comes from the recording of electrical activities of the brain with each stage showing a distinct EEG pattern
• Stage 1 of the sleep cycle:

  • light sleep
  • easily woken up
  • brain waves are slower (theta waves)
• Stage 2 of the sleep cycle:

  • Bursts of high frequency waves- 'sleep spindles'
  • easily woken
  • aware of sounds and activity
• Stage 3/4 of sleep cycle:

  • brain waves slow and increase in amplitude and wavelength- 'delta waves'
  • deep sleep
  • hard to wake up
• Stage 5 of the sleep cycle:

  • REM (rapid eye movement) sleep
  • dreams and deep sleep
  • brain is very active- almost indistinguishable from an awake brain
  • increased heart rate, blood pressure and body temp
  • eyes twitch rapidly
• STRENGTHS:

  • Provide supportive evolutionary explanations
  • Supportive research
  • Practical application
• LIMITATIONS:

  • Methodological limitations in synchronisation studies (confounding variables)
• Menstrual synchrony is advantageous as it means women would get pregnant at the same time and have babies at the same time- decreasing infant mortality. It has an evolutionary purpose
• Early synchronisation studies may have confounding variables that were not considered (stress, diets, and overall health). Women were also required to self-report their menstrual cycle which may lead to misreporting. Lacks internal validity
• Ultradian rhythms have supportive research: Dement and Kleitman (1957) measured sleep patterns of 9 adults by measuring brain waves. Alcohol and caffeine was controlled. REM was highly correlated with dreaming. Participants woken during dreaming actively recalled their dreams.
• Dement and Kleitman's study shows REM is an important component of the sleep cycle as EEGs show high levels of activity
• The treatment for SAD is phototherapy, which is exposure to bright light in the morning and evening and is thought to reset melatonin levels in those with SAD. 60% have relieved symptoms (Eastman et al, 1998).
• Phototherapy signals to the pineal gland to stop secreting melatonin, this has a coinciding effect of increasing serotonin production, relieving symptoms