The Circadian Rhythm

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The Circadian Rhythm V2 Circadian rhythms are biological rhythms which last around 24 hours. They are controlled by (internal) endogenous pacemakers and entrained by (external) exogenous zeitgebers. The sleep-wake cycle follows an endogenous pacemaker - the suprachiasmatic nucleus - which sets a cycle that lasts around 24-25 hours, although this varies between individuals (chronotype). In normal circumstances, the cycle is entrained by exogenous zeitgebers (most importantly, daylight). This ensures that the circadian rhythm runs in time with the world outside. When isolated from these time cues the cycle is 'free-running', and is controlled solely by the SCN's internal rhythm. This often leads to desynchronization with the environment outside and the rhythm of other endogenous pacemakers, and a lengthening of the sleep-wake cycle. The core body temperature (CBT) cycle is also a circadian rhythm. Our core body temperature is lowest at around 4:30am and highest at around 6:00pm. These temperature changes also entrain the sleep-wake cycle, with low temperatures signalling a time for reduced activity. This may explain why (most of us) are fast asleep at 4:30am when our core body temperature is lowest. Hormone production also follows a circadian rhythm. Cortisol is a hormone which makes us feel alert. Levels of cortisol are lowest at around midnight and highest at around 6:00am. This makes us drowsy at night - promoting sleep - and encourages us to wake in the morning. Melatonin is a hormone which induces sleepiness. Melatonin levels are highest at around midnight, also promoting sleep. Another hormone which is released mainly at night is growth hormone (GH) whose levels peak at around midnight. GH is vital for growth and body repair. Research which supports the existence of a 'free-running' internal circadian rhythm comes from the case study of a

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