Endogenous and exogenous factors in bodily rhythms WJEC PY4

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Bodily rhythms and ultradian rhythms

During the sleeping  hours we cycle in and out of dream sleep every 90mins e.g menstrual cycle. Sometimes cycles are linked to changes in our physical environment e.g shortening of day/light. It has also been noted that internal factors can affect our biological cycles. This could be known as our ‘biological clock’. 

Richter (1922) proposed that cycles are generated internally, he concluded that by studying rats. (Ethical and generalisability issue). Before this psychologists believed behaviour changed due to direct response of envioromental stimuli.

Ultradian Rhythms- occure more than once over a 24 hour period. e.g 90 min cycle of REM. Sleep is studied through the use of the EEG to measure electrical activity in the brain. Sleep has been shown to include several shorter cycles and is not a single state. Friedman & Fisher (1967) observed eating and drinking behaviour in a group of psychiatric patients over periods of 6hrs. They detected a clear 90min cycle in eating and drinking behaviour.

Some cycles are ‘invisible’ such as the release of hormones from the liver Brain systems have been identified which regulate ultradian rhythms. Lesions to nuclei in the hypothalamus disrupt or abolish them ergo it can be suggested that these nuclei have an important role to play.

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Circadian rhythms

2) Circadian Rhythms- 24 hour cycle e.g sleep wake cycle.

De Coursey (1960) found that an animal (flying squirrel) maintained in an artificial environment, with constant temperature & light, wont maintain its usual sleep/wake cycle. However in the absence of the usual cues (light/dark, temp) the rhythm can often run fast or slow.

Morris et al (1990): Human body temperature falls during the night and reaches its peak during mid-afternoon. This is associated with peaks in other functions such as heart rate & blood pressure. The same peaks occur when the daily activity cycle is reversed thus indicating that they are regulated by an internal ‘clock’. 

Folkard et al (1985) 12 P’s lived in an isolation unit for 3 weeks. They were isolated from natural light and other time cues. They agreed to get up and go to bed at set times on the clock. The clock gradually increased in speed until it was actually running on 22hrs. Only one participant was able to keep up with this pace. 

Kerkof (1985) identified ‘Larks’ who work best during the morning and ‘Owls’ who are more productive during the late evening/night period.

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Infradian rhythms

Infradian Rhythms- greater than 24 hours e.g the menstrual cycle. 

Sabbagh & Barnard (1984) - menstrual periods become synchronised. The exact cause of this is unknown; one theory = connected to the unconscious detection of pheromones secreted at various times during the menstrual cycle (Russell et al, 1980)

Russell et al (1980): Daily samples of sweat were collected from one group of women and rubbed on the upper lips of other women. The womens cycles became syncronised with that of their ‘odour donor’. This suggests that the pheremones in the sweat affected the menstrual cycle.

A characteristic of this cycle for many women is PMS this is thought to affect up to 40% of women (Choi, 1999) and results in stress, irritability, poor concentration and headaches for days before the onset of menstrual bleeding. Dalton (1964) reported that a large proportion of crimes were clustered in the pre-menstrual interval along with a rise in accidents and suicides.

In humans factors such as age, lifestyle, and individual preferences can affect the length of cycles.Fuller et al (1981) found evidence to suggest that there might be more than one circadian ‘clock’.

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Endogenous clocks & environmental factors

In order to test the relative effects of internal and external cues on a human’s circadian rhythm it is necessary to place humans in the same kind of deprivation studies that have been used on animals (ethical?) . Those where the environment is kept constant.  It has been found that if humans are placed in a situation without cues that determine the time of day they will still stick to a 24hr routine. 

Siffre (1975): spent 6 months inside a cave and discovered that his natural circadian rhythm was just over 24 hours but could change dramatically to as much as 48 hours. Does environment matter? Or can we rely on our internal sleep systems.

Similarly Aschoff & Weaver (1976) confined participants in an underground bunker and found that most maintained a circadian rhythm of between 24-29 hours. Not much change here compared to Siffre's study.

Our internal clock actually runs more slowly that the 24hr clock. Over time a person living in subterranean person will shift their cycle by about 1hr per day. After 2 weeks they will be awake when it is night on the surface and asleep when it is night. This suggests that our internal circadian clock is normally reset by environmental factors. 

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Bodily rhythms

In lower order animals (birds, reptiles) a part of the brain called the pineal gland is important in reacting to light. This sits on top of the brain and releases the hormone melatonin. Because it sits at the top of the brain it receives direct light stimulation (a small amount of light passes through the cranial bone). Although this pineal gland has its own pacemaker its light receptors influence the secretion of melatonin. Melatonin makes animals and humans feel sleepy.

In mammals there are no pineal light receptors. Instead the main ‘ clock’ in mammals appears to be located in the SCN of the hypothalamus. This is located near the optic system and receives information about light levels from the optic nerves. The less light the more melatonin is produced. 

Morgan (1995) removed the SCN from hamsters and found that their circadian rhythms disappeared suggesting that SCN is vital in order to maintain a circadian rhythm. 

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Disrupted cycles

Rusak & Morin (1976) found that lesions to the SCN of hamsters also affected their circannual breeding cycle. Male hamsters usually produce testosterone during the summer when there are long periods of daylight. But without the SCN to detect day length they produced testosterone continuously. It appears that the SCN is the main pacemaker for our daily rhythms. 

Disrupted cycles

Rapid adjustment to our circadian rhythms is required when we travel across world time zones. Small adjustments of 1-2 hours are not a problem. Moline et al (1992) suggest that it takes 1 day to adjust for every 1 hour of time zone crossed. However if you have been on a particularly long journey that has crossed many time zones then you feel what is known as jet lag.  Common symptoms of Jet Lag are: *Tiredness during ‘new’ daytime hours and an inability to sleep at night *Decreased mental performance, *Decreased physical performance, *Loss of appetite. & *Increased irritability, headaches & mental confusion  (Reilly et al, 1997)  The human body adapts more easily to gaining time (phase delay) than it does to losing time (phase advance). 

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Shift Workers

Night shift workers usually sleep less during the day than they would do at night. Shift work can result in: Sleep disturbances, Fatigue, Digestive Problems, Lack of concentration Pinel (1997) suggests that the above factors can lead to a reduction in productivity and job satisfaction and also an increase in accidents.

A Standard arrangement is 3 eight hour shifts this allows companies to maintain production for 24hrs a day. Monk & Folkard (1985) They argued that shift rotation prevents an accumulation of sleep deprivation & is beneficial to both employers and employees. 

Czeisler et al (1982) found that workers took 16 days to adjust fully to a new shift and thus recommended that shifts changed on a 21 day rotation i.e. once every three weeks. They also suggested that workers should move forward rather than backwards when changing shifts. This is inconvinient for both employers and employee's as we live in a demanding envioroment where sleep is not as significantly regarded when given shifts.

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Melatonin is a hormone released by the pineal gland and is thought to be vital in sleep regulation in humans. Melatonin has also been linked with SAD. The darkness of winter can increase melatonin output leading to tiredness & depression.

Melatonin plays a crucial role in the feeling of Jet Lag. It is mainly released at night and after a long flight the release of melatonin remains on the day/night pattern of the departure country for several days. In the USA a synthetic version of melatonin is marketed as a cure for jet lag and insomnia however this is banned in the UK because not enough is known about its effects.

Blakemore (1988) did show that giving melatonin to jet lagged volunteers was more successful at relieving symptoms than a placebo suggesting that melatonin prepares the body for sleep easier than a psychological method. However melatonin has also been shown to affect other cycles, such as the reproductive cycle, and thus should be used with caution.

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