Circadian rhythms

• Physically, the circadian clock is located in the suprachiasmatic nucleus in the hypothalamus of the brain, one in each brain hemisphere.
• 1. Core temperature usually reaches its minimum around 4:30-5:00 am in the morning in human adults
  2. Melatonin (normally absent during daylight hours) typically begins to be produced around 8:00-9:00 pm at night and stops at 7:00-8:00 in the morning
  3. The deepest tendency to sleepiness occurs in the middle of the night, around 2:00-3:00 am along with a shorter and shallower period of sleepiness about 12 hours later.
• Aschoff and Wever (1976) found that participants who spent 4 weeks in a bunker without natural light displayed circadian rhythms of around 24-25 hours. Natural rhythm shortened by natural daylight.
• Poor control in studies. Cave studies - poor control over artificial light. Research by Czeisler found that dim artificial lighting could adjust the circadian rhythm by 22-28 hours.
• Application to pharmacokinetics. By understanding circadian rhythms, it can help scientists work out which times are best for medicinal administration. For example, the risk of a heart attack is greatest in the early morning.
• Support from cave studies. Siffre spent two months in the caves of the southern Alps deprived of sight and sound.
• Issues with case evidence. The Siffre study was only conducted with one participant. When he went back into the cave at 60 years old, he found that his body clock followed more of a 48 hour cycle.
• Shift work has been found to lead to desynchronization of circadian rhythms. Research shows that night shift workers experience lapse of concentration at 6 am, increasing likelihood of accidents etc.
• Reinberg examined a woman who spent 3 months in a cave with only a small lamp to provide light. Noted that her menstrual cycle shorterned from 28 to 25.7 days.
• STAGES OF SLEEP

  1. 4-5%. Light sleep. Muscle activity slows down. Occasional muscle twitching.
  2. 45-55%. Breathing pattern and heart rate slows. Slight decrease in body temperature.
  3. 4-6%. Deep sleep begins. Brain sleep begins. Brain begins to generate slow delta waves.
  4. 12-15%. Very deep sleep. Rhythmic breathing. Limited muscle activity. Brain produces delta waves.
  5. 20-25%. Rapid eye movement (REM). Brainwaves speed up and dreaming occurs. Muscles relax and heart rate increases. Breathing is rapid and shallow.
• Research into SAD has produced great practical applications for sufferers. Phototherapy works by using stimulating light in the morning and evening, resetting melatonin levels. Relieves symptoms in up to 60% of sufferers (Eastman).
• There is evidence to support the separate and distinct sleep stages. Dement and Kleitman monitored sleep patterns of 9 adult participants, correlating REM with dreaming.
• DECOURSAY et al

  • From late April 1997 through October 1998
  • 30 chipmunks with suprachiasmatic nucleus targeted lesions
  • Restless behaviour in burrows.
  • During the first 80 days, significantly more chipmunks with the lesion were predated by weasels.
• Damiola (2000) demonstrated that changing the feeding patterns in mice could alter the circadian rhythm of liver cells by up to 12 hours. Suprachiasmatic nucleus might be reductionist.
• Miles (1977) - young man blind from birth with circadian rhythm of 24.9 hours - could not change his sleep pattern despite social cues. Exogenous zeitgebers exaggerated?