circadian rhythms

• Circadian rhythms

  A biological rhythm that lasts around 24 hours. We have circadian rhythm for regulating sleep and wake, the release of hormones, and varying our body temperature and blood pressure.
• Endogenous pacemakers


  Internal body clocks that keep biological processes to time.
• Exogenous zietgebers


  External cues that entrain internal body clocks to match the environment.
• Sleep-wake cycle


  The pattern of sleep and wake that repeats after 24 hours. We are aware of conflicts between endogenous pacemakers and exogenous zietgebers like light and social cues in situations like jet lag. The endogenous pacemaker will be entrained by the exogenous zietgebers to the new time zone, but it can take several days.
• Suprachiasmatic nucleus


  The endogenous pacemaker for the sleep-wake cycle, known as the master clock. The SCN is part of the hypothalamus within the limbic system. When light is detected by the SCN it sends a signal to the pineal gland, stopping the production of melatonin, a hormone that results in a sleep response.
• + Supporting evidence


  E - Siffre investigated how a lack of light may interrupt circadian rhythms by spending two months in a cave without access to daylight. His circadian rhythm was notably disrupted, settling into a sleep/wake cycle of around 25 hours.

  E - This suggests that environmental cues have an influence on circadian rhythms.

  L - However, the influence was only small, suggesting other factors may play a role.
• / Control in studies is limited


  E - Siffre was deprived of natural light but still had access to artificial light. This artificial light could have influenced circadian rhythms.

  E - Other studies that controlled for artificial light found the circadian rhythm was closer to 24 hours.

  L - Therefore, the lack of control where participants are not always deprived of all light means that the influence of circadian rhythms may not accurately measured.
• / Case studies and small sample sizes


  E - Studies which investigate the sleep wake cycle only involve small samples of participants or even single individuals (as with Siffre).

  E - The individuals may not be representative of a wider population

  L - Therfore, this limits our ability to generalise findings to others.
• + Practical applications- Drug treatments


  E - Our understanding of sleep/wake cycles and circadian rhythms has allowed us to apply this to pharmokinetics (how well drugs are absorbed and distributed in the body). Drugs are most effective in certain times of day and night.

  E - This helps with the development of guidelines on timings of drug doses for medications.

  L - Therefore, our understanding on circadian rhythms has allowed for effective treatments to be made.