Circadian Rhythms

• The circadian rhythm is known as the ‘body clock’
• Circadian rhythms operate on a 24 hour cycle which is reset by levels of light (sleep-wake cycle)
• Environmental events can also reset the body clock - these are called exogenous zeitgebers e.g.

  • changes in light levels
  • reading by lamp at night instead of main light
• The suprachiasmatic nuclei (SCN) in the hypothalamus detects the level of light present

  • the SCN then uses this info to coordinate the activity of the entire circadian system by either triggering sleep or telling the body to wake up
• Melatonin plays a role in triggering sleep by responding to a decrease in the levels of light (explains why you may feel sleepy in a darkened cinema even in the middle of the day)
• Melatonin secretion correlates with increased feelings of sleepiness about 2 hours before someone’s regular bedtime

  • very hard for sleep to occur before this 2 hours window
• Once melatonin has been secreted, there is a point at which the ‘sleep gate’ opens and sleep is inevitable (hypnagogic stage)
• • Melatonin levels fall during daylight hours
  • Melatonin levels begin to rise as daylight decreases
  • Melatonin levels peak during darkness and night hours
• Shift work and jet lag have been found to lead to desynchronisation of circadian rhythms and can lead to adverse cognitive and psychological efffects e.g.

  • memory lapses
  • poor concentration
  • issues with fertility
  • increased risk of cancer, heart diabetes and diabetes