Endogenous rhythms are also related to the pineal gland. The pineal gland also contains light sensitive cells so when light levels drop, melatonin is produces and as light levels increase melatonin production is inhibited. Melatonin caused the raphe nucleus to produce serotonin which in turn inhibits the activity of the reticular activating system. This mechanism is involved in wakefulness when it shuts down; nerve signals to the limbs are repressed so movement decreases and a sleep state are reached.
Supporting evidence comes from Potocki who found that levels of melatonin are inverted for those who suffer from difficulty to fall asleep. Taking supplements of melatonin at night can help with symptoms, thus supporting role of melatonin in the sleep/wake cycle. As this research is based on humans, it is likely to be valid and reliable.
Exogenous zeitgebers are time-givers that are externally caused and entrain biological rhythms. The process of resetting the biological clock with exogenous zeitgebers is known as entrainment. The opposite of entrainment is ‘free-running’ where the biological clock operates in the absence of any exogenous cues.
Until fairly recently, biologists thought that social cues were the main zeitgebers for human circadian rhythms due to us eating meals at socially determined mealtimes and going to bed and waking up at times designated times as appropriate for our age. Our daily rhythms appeared to be entrained by social convention, not internal biology. However it has been recognised that light is the dominant zeitgebers in humans as light can reset the body’s main pacemakers, the SCN. It can also reset other oscillators located throughout the body. Miles (1977) studied a blind man who had a 24.9 hour circadian rhythm despite being exposed to a variety of zeitgebers that should have set his ‘clock’ to 24 hours. He had stimulants and sedatives to coordinate his sleep/wake cycle with the rest of the world. This demonstrates that light is the dominant time-giver. However, light cues can be overcome. Luce and Segal found that people who live within the Arctic Cycle still sleep for about 7 hours, despite the fact that during the summer months the Sun never sets. In this case, social cues are dominant.
Criticisms of Miles’ research however is that as it was a case study, we cannot generalise these findings to the rest of the population so they lack ecological validity as individual differences may have affected the results for example.
However it is understood that all parts of the body produce their own oscillating rhythms, some of which are not primarily reset by light. For example the zeitgebers for cells in the liver and heart is likely to be mealtimes because these cells are reset by eating.
Biological rhythms can also be entrained by temperature. For example leaves on deciduous trees change colour and drop off because of changes in temperature as well as day length. Temperature is also a factor in the onset of hibernation. However there is no evidence to suggest that temperature affects human biological rhythms and we cannot apply animal’s findings to humans.
In research by Pengelly and Fisher, squirrels were put in highly controlled environments with artificial light for 12 hours a day at a constant temperature. It was found that even though external cues were unchanging, squirrels still hibernated normally. Therefore squirrels have endogenous circadian rhythms for hibernation, not dependant on exogenous factors. Although we cannot apply these findings to humans we can check them with human findings. When Michel Siffre spent two months living in total isolation in a cave without access to exogenous cues, his average day “day” lasted a little over 24 hours showing that we do have internal clocks governing our sleep/wake cycle.
To problem with researching light as an exogenous factor is that often artificial light is used. In early studies, participants were exposed to artificial lighting but it was assumed that this would not be bright enough to entrain circadian rhythms. Recent research has shown that in general, artificial light does have an effect and so if dim lighting does reset the biological clock, then living in an artificially lit world may have negative consequences.
The menstrual cycle is an example of an infradian rhythm and although it is normally governed by an endogenous system – the release of hormones under the control of the pituitary gland, it can be controlled by exogenous cues. Research has shown that when several women live together, they tend to menstruate at the same time every month, suggesting the synchronising of menstrual cycles can be affected by pheromones which are chemicals released in sweat. However some researchers have replicated these studies and were not able to replicate these findings and so there is dispute to as how reliable his theory is.
We need pacemakers, otherwise there would be problems. If animal’s rhythm solely relied on environmental cues we might sleep most of the day in winter exogenous zeitgebers allow animals to respond to changes in the environment, without them an animal might be awake at unsuitable times.
