Another endogenous pacemaker is the pineal gland and melatonin.
This gland contains light receptors that respond to external light via the thin layer or skull that lies above the gland. These receptors influence the activity of neurons in the gland. Melatonin acts on many organs and glands and is especially responsible for sleep/wake rhythms via the mechanism of the brainstem.
Schochat’s study on the involvement of melatonin in the sleep waking cycle is some of the strongest evidence for this area of research. Their six male participants spent 29 hours between 7am one day to noon the following day in the sleep lab. In this time they spent 7 out of every 20 minutes lying in bed in a darkened room trying to sleep. This allowed them to measure sleep propensity and tendency to sleep at different times during the day. The period of greatest sleep propensity is known as the “sleep gate,” and starts in the late evening. The period of lowest propensity occurs in early evening shortly before the sleep gate.
Blood samples taken up to 3 times an hour during the 29-hour session measured the levels of melatonin. Results showed a close relationship between the circadian rhythms of sleep propensity and melatonin.
This study was only carried out on males and so cannot therefore be generalised to females. The sample size is fairly small so this too could affect generalisability and doubt the validity of the findings.
However, Schochat et al’s research in the sleep lab took place in a well-controlled environment. The greater control of confounding variables gives confidence in the finding that melatonin and sleep propensity are associated.
This type of control could however be criticised for lacking realism. The experiment was in an unrealistic setting and not under everyday circumstances.
Schochat’s relationship is correlational not causal and so we cannot conclude that melatonin causes sleep propensity. However, the research is supported by the fact that insomniacs treated with melatonin do find it easier to get to sleep.
Much of the research into endogenous pacemakers is case studies or small samples, which causes population validity to be a weakness.
Exogenous factors are ones which are based externally to the organism.
Zeitgebers are external events which partially determine biological rhythms.
Examples of these can be the environment e.g. temperature, light etc. Another example would be social cues and obligations such as meals, school, jobs etc.
Until recently it was thought social cues were the main Zeitgebers for human circadian rhythms. However, light has been recently recognised as the dominant Zeitgebers in humans.
Campbell and Murphy (1998) demonstrated that participants given regular light exposure on the backs of their knees had changes in their circadian rhythms in line with light/dark they were exposed to. However, it is not clear how this information would get to the SCN.
One criticism is that this may not be the same case for a person that is blind. However, Miles et al (1977) documented a young man who was blind from birth. He had a strong 24.9 circadian rhythm despite the fact he was exposed to a variety of Zeitgebers such as clocks and radios. He had to use stimulants and sedatives to co-ordinate his sleep-wake cycle with the rest of the world due to problems in resetting his biological clock. This demonstrates that light really is the dominant Zeitgeber.
Other external cues are also important. The circadian rhythms of people living within the Arctic Circle illustrate the influences of internal and external mechanisms as they still sleep for about 7 hours despite that the sun never sets.
It is adaptive to be able to respond to environmental chance which is why animals are sensitive to these Zeitgebers.
Lindsay Graham
U63
