Biological rhythyms

• Biological rhythm


  A change in the body's processes in response to environmental changes
• Types of biological rhythms

  • Circadian
  • Infradian
  • Ultradian
• Circadian rhythms


  • Follow a 24-hour cycle
  • Example: sleep-wake cycle
• Exogenous zeitgeber (EZ)


  Environmental cue that contributes to biological rhythms (e.g. daylight)
• Siffre spent 6 months underground, living in a cave completely cut off from natural light (EZ's). Siffre's body seemed to have a preference for a 25 hour rather than a 24-hour cycle. This implies that circadian rhythms are mainly controlled by endogenous pacemakers (EP's) rather than exogenous zeitgebers (EZ's)
• Aschoff and Wever (1976) found that participants who spent 4 weeks in a bunker without natural light showed circadian rhythms of 24-25 hours, except one participant who went up to 29 hours.
• Folkard et al (1985) found that when participants were deprived of sunlight for 3 weeks, and the length of day was manipulated by the researchers to 22 hours rather than 24, all but one participant maintained a consistent 24-hour circadian rhythm.This suggests the strength of the body's sleep/wake cycle, as it resisted environmental changes
• Body temperature circadian rhythm

  • Lowest in early morning (36oC at 4:30 am)
  • Highest in early evening (38oC at 6 pm)
  • Sleep occurs when temperature starts to drop, and rises towards end of sleep cycle
• Duffy et al (2001) found that ‘morning people’ prefer to rise and go to bed early whereas ‘evening people’ prefer to wake and go to bed later. This demonstrates that there may be innate individual differences in circadian rhythms, which suggests that researchers should focus on these differences during investigations.
• This demonstrates that there may be innate individual differences in circadian rhythms
• Czeisler et al (1982) found that employees whose shifts were stable over 21 days or more had greater employee satisfaction, improved health estimates, and were less likely to leave their job than employees whose work schedules changed every week
• This is likely because keeping shift schedules over 21 days or more allowed workers' circadian rhythms to adjust to their work schedules
• Buhr et al. (2010) found that fluctuations in temperature set the timing of cells in the body, and that information about light levels is transformed into neural messages that set the body's temperature
• This shows that circadian rhythms are controlled and affected by several different factors, and suggests that a more holistic approach to research might be preferable
• There are methodological concerns to research in this area as it often uses small sample sizes (only 1, in the case of Siffre), so generalisation may be difficult
• Participants had access to artificial light, which could have acted as a confounding variable
• The research could be criticised for being reductionist as it only considers a singular biological mechanism and fails to consider the other widely divergent viewpoints
• Infradian rhythms


  • Occur less than once a day
  • Examples: menstruation (monthly), hibernation (yearly)
• Female menstrual cycle


  Regulated by hormones that either promote ovulation or stimulate the uterus for fertilisation
• Ovulation occurs roughly halfway through the menstrual cycle when oestrogen levels are at their highest, and usually lasts for 16-32 hours
• After the ovulatory phase, progesterone levels increase in preparation for the possible implantation of an embryo in the uterus
• The usual menstrual cycle is around 28 days, but varies with some women experiencing a short cycle of 23 days and others experiencing longer cycles of up to 36 days
• Seasonal affective disorder (SAD)


  Infradian rhythm governed by a yearly cycle, where some people become depressed in the winter
• Lack of light during the winter months results in a longer period of melatonin secretion, which has been linked to the depressive symptoms of SAD
• Russell et al. (1980) found that female menstrual cycles became synchronised with other females through odour exposure
• This suggests that the synchronisation of menstrual cycles can be affected by pheromones, which have an effect on people nearby rather than on the person producing them
• Evolutionary psychologists claim that the synchronised menstrual cycle provides an evolutionary advantage for groups of women, as the synchronisation of pregnancies means that childcare can be shared among multiple mothers who have children at the same time
• Light box treatment for SAD has led to a relief in symptoms for around 60% of sufferers
• Terman (1988) found that the rate of SAD is more common in Northern countries where the winter nights are longer
• These results suggest that SAD is in part affected by light (exogenous zeitgeber) that results in increased levels of melatonin
• Ultradian rhythms


  • Occur more than once a day
  • Example: different stages of sleep
• REM (rapid eye movement) sleep


  Controlled by the locus coeruleus (LC) which produces noradrenaline and acetylcholine
• NREM (non-rapid eye movement) sleep

  Subdivided into stages 1-4, with stages 3 and 4 being 'slow wave sleep'
• Demont and Kleitman (1957) used an EEG to investigate brain activity during sleep over the course of a night, and found that participants reported dreams when woken during REM sleep
• Tucker et al. (2007) found significant differences between participants in terms of the duration of each stage of sleep, particularly stages 3 and 4
• This demonstrates that there may be innate individual differences in ultradian rhythms
• Duffy et al (2001) found that ‘morning people’ prefer to rise and go to bed early whereas ‘evening people’ prefer to wake and go to bed later. This demonstrates that there may be innate individual differences in circadian rhythms, which suggests that researchers should focus on these differences during investigations.