circadian rhythms

• our circadian rhythms are driven by our body clocks found in all of the cells of the body and synchronised by the master circadian pacemaker and suprachiasmatic nuclei (SCN) found in the hypothalamus
• this pacemaker must constantly be reset so that our bodies are in synchrony w/ the outside world, light provides the primary input to this system, setting the body clock to the correct time in a process termed phototentrainment
• post-mortem examinations L: in mammals light-sensitive cells within the eye act as brightness detectors sending messages about environmental light levels direct to the SCN
• post-mortem examinations L: the SCN then uses this information to coordinate the activity of the entire circadian rhythm, the most familiar of the circadian rhythms subject to the entrainment process is the sleep-wake cycle
• 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 sleep-wake cycle: the circadian rhythm also dips and rises at different times of the day so our strongest sleep drive usually occurs in 2 'dips' between 2-4am and between 1-3pm (the 'post-lunch dip')
• the sleep-wake cycle: the sleepiness we experience during these circadian dips is less intense if we have had sufficient sleep and more intense when we are sleep deprived
• the sleep-wake cycle: sleep and wakefulness are not determines by the circadian rhythm alone but are also under 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
• the sleep-wake cycle: this homeostatic drive for sleep increases gradually throughout the day reaching its maximum in the late evening when most people fall asleep, therefore the circadian system keeps us awake as long as there is daylight prompting us to sleep as it becomes dark
• the sleep-wake cycle: the homeostatic system tends to make up sleepier as time goes on throughout the waking period regardless of whether it is night or day, the internal circadian 'clock' is described as 'free-running' i.e. it will maintain a cycle of about 24-25 hours even in the absence of external cues
• the sleep-wake cycle: the circadian system is however intolerant of any major alterations in sleep and wake schedules (e.g. through jet travel, shift work) because this causes the biological clock (and the internal physiological systems that are dependent on this) to become completely out of balance
• core body temperature: core body temperate is one of the best indicators of the circadian rhythm, it is at its lowest (about 36 degrees Celsius) at about 4:30am and at its highest (about 38 degrees Celsius) at about 6pm
• core body temperature: during the normal circadian rhythm, sleep occurs when the core temperature begins to drop and body temperature starts to rise during the last ours of sleep promoting a feeling of alertness in the morning
• core body temperature: 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 e.g. the production and release of melatonin from the pineal gland in the brain follows a circadian rhythm w/ peak levels occurring during the hours of darkness
• hormone production: 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
• case study: evidence for a 'free-running' circadian rhythm comes from a series of studies conducted on the French cave explorer Michel Siffre, on several occasions Siffre has subjected himself to long periods of time living underground he had no external cues to guide his rhythms = no daylight, no clocks or radio
• case study: he simply woke, ate and slept when he felt it was appropriate to do so, the only thing influencing his behaviour was his internal body clock i.e. his 'free-running' circadian rhythms
• case study: after his 1st underground stay of 61 days in the southern Alps in 1962 he resurfaced on 17th September believing the date was really 20th August
• case study: on the 2nd occasion he spent 6 months in a cave in Texas, his natural circadian rhythm settled down to just over 24 hours but w/ some dramatic variations
• case study: on his final underground stay in 1999 he was interested in the effects of ageing on circadian rhythms (by this time he was 60 years old) he found that his body clock ticked more slowly compared to when he was a young man sometimes stretching his circadian rhythms to 48 hours
• evaluation S: research suggests there are individual differences in circadian rhythms so according to Duffy et al (2001) they would explain why some people prefer to rise early and go to bed early (about 6am and 10pm) whereas others prefers to wake and go to bed later (10am and 1am)
• evaluation L: Buhr et al (2010) believe that it temperature that actually controls our body clock rather than light, body temperature fluctuates on a 24 hours circadian rhythm and even small changes in the body temperature can send a powerful signal to our body clock, suggesting that temperature may be more important than light in setting circadian rhythm
• evaluation S: one real-world application of circadian rhythms is chronotherapeutics - the study of how timing affects drug treatments = the specific time that patients take their medication is very important as it can have a significant impact on treatment success, as a result chronotherapeutics medications have been developed w/ novel drug delivery system, these medications can be administered before the person goes to sleep at 10pm but the actual drug is not released until the vulnerable period of 6am to noon
• evaluation S: Hughes (1977) tested the circadian hormones release in 4 participants stationed at the British Antarctic Station, the findings of this study suggests that the extremes of daylight found in polar regions of the world may be responsible for variation in circadian hormone release, however other research using scientific communities in the Arctic who would be subject to similar prolonged winter darkness found no such disruption of cortisol release patterns
• evaluation L: other studies have shown big individual differences in the onset and duration of rhythms suggesting that a purely biological explanation of the rhythm is too reductionist as other factors play a part