Endogenous pacemakers
A study done on rats by Stephan et all where they damaged the SCN cells by removing them to see the effect it had on circadian rhythms. They put one group in 12 hours of light and the other in 12 hours of darkness. They found the rats with no SCN drank more and were more active in light than darkness. They concluded that if the SCN is damaged the circadian rhythm goes. A study that also supports this is the study done by Morgan who used hamsters but they transplanted SCN cells from mutant hamsters, which followed a 20-hour cycle into a hamster with a normal 24 hour cycle. They found that the normal hamster took on the 20-hour cycle. This is backed up by anatomical studies which have shown the presence of the SCN just above the optic nerve which is part of the eyes and we know that the eyes play a big part in the sleep wake cycle and are effected by light. The non-human animal studies are supported by brain tumours that have damaged the SCN have caused messed up circadian rhythms. This shows that SCN has the effect on both types of animals this means the studies results can be applied to human and non-human animals. However both studies done on animals were very unethical as the procedures used to damage or remove the SCN cells caused death and permanent damage to the animals causing them physical damage, which is not reversible. However the scientific methodology is good as it was done in a lab so it is very scientific, all variables are controlled and this means that the results can be replicated easily. However they can no be applied to humans as it would be very unethical cause it is not reversible and would probably lead to death. The species used were all rodents so would not be able to generalise to the rest of the animal kingdom but further studies have been done on lots of species of animals to see if it occurs on all animals. It can