Circadian Rhythms

• The Nature of Circadian Rhythms

  Circadian rhythms are driven by our body clocks, found in all our cells of the body, and synchronised by the master circadian pacemaker, the suprachiasmatic nuclei (SCN) found in the hypothalamus. This pacemaker must constantly be reset so that our bodies are in synchrony with the outside world. Light provides the primary input to this system, setting the body clock to the correct time in a process termed photoentrainment.
• The sleep-wake cycle


  The circadian rhythm not only dictates when we should be sleeping, but also when we should be awake. Light and darkness are the external signals that determine when we feel the need to sleep and when to wake up. The circadian rhythm also dips and rises at different times of the day, so our strongest sleep drive usually occurs in two 'dips', between 2-4 am and between 1-3 pm. The sleepiness we experience during these dips is less intense if we have had sufficient sleep.
• Homeostatic control


  When we have been awake for a long period of time, homeostasis tells us that the need for sleep is increasing because of the amount of energy used up during wakefulness. This homeostatic drive for sleep increases gradually throughout the day, reaching its maximum in the late evening when most people fall asleep.
• What are the 2 other circadian rhythms I need to know.

  Core body temperature and Hormone production
• Core body temperature


  It is at its lowest at about 4:30 am and at its highest around 6pm. During the normal circadian rhythm, sleep occurs when the core temperature begins to drop, and body temperature starts to rise during the last few hours of sleep, promoting a feeling of alertness in the morning. A small drop in body temperature also occurs in most people between 2pm and 4pm, which may explain why many people feel sleepy in the early afternoon
• Hormone production


  Hormone release follows a circadian rhythm. For example, the production of melatonin from the pineal gland in the brain follows a circadian rhythm with peak levels occurring during the hours of darkness. By activating chemical receptors in the brain, melatonin encourages feelings of sleep. When it is dark, more melatonin is produced and when it is light again, the production of melatonin drops and the person wakes.
• Research suggests there are individual difference in circadian rhythms.

  One is the cycle length. Research has found that circadian cycles can vary from 13 to 65 hours. The other type of individual difference relates to cycle onset - individuals appear to be innately different in terms of when their circadian rhythms reach peak.
• Case study for circadian rhythms

  Siffre subjected himself to long periods of time living underground in order to study his own circadian rhythms. While living underground he had no external cues to guide his rhythm. He found on some occasions his body clock was close to 24 hours but he also found that his body clock got slower as he aged, sometimes stretching to 48 hours.