circadian rhythms

circadian rhythms

cycles that occur once every 24 hours, e.g. sleep-wake cycle.

endogenous pacemakers

• an internal body clock that sets many of our bodily rhythms, including sleep. The internal body clock that has an effect on when we sleep and when we are awake is the suprachiasmatic nucleus (SCN).

• main endogenous pacemaker is the SCN or biological clock, which is a bundle of nerves located in the hypothalamus of the brain, above the optic area (optic nerve and optic chiasm).
  it can receive information about light directly, and passes the information about day length/light to the pineal gland.

• based on this information, the pineal gland will release melatonin, which is a hormone that makes us feel sleepy.

• in the night, the pineal gland produces more melatonin, and in the day when there is more daylight, less melatonin. The SCN is therefore to a degree regulated by the light from our outside world.

• however, even in the absence of any light (trapped in a cave), the SCN generates a rhythm related to its production of protein. When it reaches a certain level of protein it passes a message to the pineal gland and melatonin will still be released or inhibited.

• although daylight influences the SCN it’s not absolutely essential, e.g. a person who’s blind still has a sleep/wake cycle regardless of light input.

exogenous zeitgebers

• the most influential exogenous zeitgeber is light, and it’s an important factor in our environment that ‘resets’ our biological clocks, this is called entrainment.
  low levels of light go into the retina, then via the optic area to the SCN, which sends signals to the pineal gland, which then releases melatonin and induces sleep.

• high levels of light go into the retina, then via the optic area to the SCN, which sends signals to the pineal gland, which then inhibits the release of melatonin and induces wakefulness.

endogenous pacemakers supporting research - Siffre’s cave study (1962)

• spent two months living in complete isolation in a cave to study the effects on his own circadian rhythm. He was deprived of natural light, a clock, a calendar and sound, but had access to adequate food and drink.

• he slept and ate only when his body ‘told him to,’ therefore, the only influence was his internal body clock (endogenous pacemaker).

• he re-surfaced in mid-September 1962 believing it to be mid-August, a month earlier than it was. His lack of external cues made him feel a day was actually longer than it was, and fewer days had passed in total.

• a decade later he performed a similar feat for six months in a cave in Texas.

• in each case, his ‘free running’ circadian rhythm settled to around 25 hours, just beyond the usual 24 hours, showing the power of the SCN because even without external signals such as daylight, he was able to maintain an almost accurate circadian rhythm.

• however, because it was 25 hours, it provides some support for exogenous zeitgebers as it shows that our circadian rhythm adapts to light levels and exogenous zeitgebers.

Aschoff and Wever (1976)

• asked a group of participants to spend four weeks in a WWII bunker.

• participants were shielded from natural light (no windows), temperature changes or other environmental cues, but they had access to artificial light and could switch it on/off. Similar to Siffre, they displayed a circadian rhythm of approximately 25 hours, but one participant extended to 29 hours.

• these studies suggest the ‘natural’ sleep/wake cycle may be slightly longer than 24 hours but we use natural light to entrain (adjust) our pacemakers associated with the 24 hour clock.

Simon Folkard et al (1985)

• studied a group of 12 participants who lived in a dark cave for 3 weeks, isolating them from natural light.

• the researchers manipulated the clock and participants would go to bed when the clock read 11.45pm and wake up when it read 7.45am.

• over the course of the study, the researchers sped up the clock without participants knowing so what they believed was a normal 24 hour day, was actually only 22 hours.

• only one of the participants could adjust comfortably to the new length day/sleep-wake cycle.

• this suggests the existence of a strong free-running circadian rhythm that cannot be easily overridden by changes in the external environment.

cave studies - strengths

useful applications - better understanding of the consequences of disrupted circadian rhythms in night shift work. Night workers can experience reduced concentration around 6am, making mistakes and accidents more likely. Poor health has also been linked with night shifts. This highlights economic implications and how changes in shift work patterns could help workers stay healthy and manage productivity.

cave studies - weaknesses

• generalisability - small sample size - tends to involve small sample sizes, and in the case of Siffre, only one person. The participants may not be representative, which limits the degree to which the research can be generalised applied to the wider population.

• internal validity/contradicting research - confounding variables - although participants were derived of natural light, in Aschoff and Wever’s study they still had access to artificial light, and Siffre would turn on a lamp every time he woke up which remained on until he went to bed. They assumed artificial light would have no effect on circadian rhythms, however Czeisler suggests that artificial light can have an influence. This means artificial light could be a confounding variable and affect the validity of the results.

• individual differences - generalisability - individual cycles can vary as some have a preference for going to bed early and rising early whereas others prefer the opposite. There are also age differences in sleep/wake patterns and other individuals may have innate differences in their cycle length, making generalisations difficult.

SCN supporting research - Ralph’s hamster study (1990)

• aim - to show the SCN generates the circadian rhythm in mammals.

• mutant hamsters were bred so that they had a circadian rhythm of 20 hours rather than 24. The SCN cells from these abnormal hamsters were removed and transplanted into the brains of normal hamsters with a 24 hour circadian rhythm.

• the normal hamsters began to adopt the same abnormal circadian rhythm as their 20 hour donor.

• when normal hamsters with nocturnal patterns of activity had their SCNs replaced with SCNs from mutated hamsters which slept through the night and were active during the day, the hamsters followed the new daytime activities of the donor’s patterns.

• suggests the transplanted SCN had imposed its pattern onto the hamsters and shows the significance of the SCN and how endogenous pacemakers are important for biological rhythms.

Ralph (1990) - weakness - animal studies

• ethics - hamsters could not give informed consent and were harmed during the study.

• generalisability - difficult to generalise to humans because human brains are more complex than hamster’s brains.

exogenous zeitgebers supporting research - Campbell and Murphy (1998)

• showed that light may be detected by skin receptor sites on the body, even when the same info is not received by the eyes.

• 15 participants were woken up at various times and a light pad was shone on the back of their knees.

• the researchers found a change in their sleep/wake cycle of up to 3 hours in some cases.

• suggests that light is a powerful exogenous zeitgeber that doesn’t need to rely on the eyes to exert influence on the brain.

Campbell and Murphy - weakness

• reliability - methodological issues - findings from the Campbell and Murphy study have yet to be replicated.

• critics have suggested that participants may have been exposed to a limited amount of light to their eyes which could be a major confounding variable and affect the validity of the results.