Circadian Rhythms - The Sleep-Wake Cycle
What is a circadian rhythm?
- A circadian rhythm is any biological process that displays an endogenous cycle (internal cycle) of about 24 hours.
The sleep-wake cycle is an example of a circadian rhythm. There are external cues (i.e. light and darkness or the time) that help us know when to go to bed and wake up and these are known as exogenous zeitgebers. However, psychologists have been interested in what happens when you take away the external factors and leave a person to rely fully on their internal cues - known as their endogenous pacemaker. Studies that aim to do this are called temporal isolation studies, one of the most memorable being conducted by Michel Siffre who spent 6 months in a cave with no natural light or indication of time. It was found that Michel's sleep-wake cycle settled to just over 24 hours however it did occasionally shift to as much as 48 hours. Therefore Michel's endogenous pacemaker did (generally) keep his sleep-wake cycle at a fairly consistent circadian cycle suggesting that external cues do not play much of a role in circadian rhythms.
A major flaw in the research methodology of Siffre's experiment was the fact that he had a head lamp which exposed him to artificial light. It was thought then that dim, artificial light would not affect circadian rhythms but it has recently been found that the sleep-wake cycle can be altered cut down to 22 hours or up to 29 hours using only artificial light. This suggests that the results of the study are not valid because the head lamp could have been an external cue that helped to regulate Siffre's sleep pattern.
Another study into the role of exogenous and endogenous factors in the maintenance of circadian cycles was that of Folkard who conduced an experiment to see if external cues could override a person's internal pacemaker. A group of 12 people lived in a cave for 3 weeks, isolated from light and other exogenous zeitgebers, however they did have a clock. The clock initially ran normally but gradually started to get quicker until it was indicating the passing of 24 hours but really only 22 hours had passed. The participants agreed to go to bed at 11.45 and wake up at 7.45. It was found that the participants sleep-wake cycle continued to follow a 24 hour cycle rather than change to the 22 hour external clock. This suggests that endogenous pacemakers are the dominant factor in regulating circadian rhythms and external cues do not play much of a role.
The above research has all been criticised for not taking into account individual differences between the participants. For example, research has found that circadian rhythms can vary a lot in different people (from 13 to 65 hours). Also different people have different times where their circadian rhythm reaches it's peak, for example some people are 'morning people' who prefer to get up early and go to bed early, whereas others are 'evening people' who prefer to go to bed late and wake up late. The research did not consider these things when drawing conclusions from the results and thus lose validity.
I keep referring to endogenous pacemakers as an 'internal body clock', but how does your body actually regulate your circadian rhythms? In mammals the main endogenous pacemaker is a small cluster of nerve cells within the hypothalamus in the brain known as the suprachiasmatic nucleus (SCN). IT is located in the optic chiasm (a place where the optic nerves from each eye cross over). In fact the SCN is a pair of structures - one in each hemisphere of the brain and each one of these is further divided into a ventral and a dorsal SCN. The SCN obtains information about light from the eye via the optic nerve and if it is dark outside will send signals to the pineal gland directing it to release more melatonin. Melatonin induces sleep by inhibiting the brain mechanisms that promote wakefulness.
Research Into The SCN
Morgan (1995) bred hamsters to have circadian sleep-wake cycles of 22 hours rather than 24 hours. The hamsters SCN was then taken out and put into another 'normal' hamster. It was found that the normal hamsters adopted the 22 hour cycle thus showing that the SCN does in fact dictate an animal's circadian rhythms.
Another study involved completely removing the SCN in 30 chipmunks and then returning them to their natural habitat. The chipmunks were observed alongside 24 surgical controls and 20 intact controls. It was found that after 80 days significantly more of the chipmunks who had their SCNs removed had been killed by weasels. This was because without their SCN the chipmunks remained awake in their burrows and the weasels could hear them and thus able to locate them.
Core Body Temperature
Another circadian rhythm is core body temperature. It is lowest at around 4.30 am (36°C) and highest at around 6.00 pm (38°C). This circadian cycle has been linked to cognitive abilities, for example it was found that 12 to 13 year old participants who had stories read to them in the afternoon retained about 8% more meaningful information and showed superior recall and comprehension compared to the participants who had stories read to them in the morning. Another study found that performance on an IQ test was better in the afternoon than the morning. This suggests that long-term recall is better when the core body temperature is at it's highest.
There has also been research that found that participants had lowered cognitive abilities when their core body temperature was lowered by placing them in cold water. However, this research has been opposed by a field experiment that found that there is no correlation between core temperature and cognitive performance. These findings could have occurred because in lab experiments the conditions are very unrealistic (i.e. lack mundane realism) and therefore cannot be generalised to real life situations.