This essay will outline and evaluate the role that endogenous pacemakers (ep’s) or internal clocks have on our biological cycles. It will also consider exogenous zeitgebers (ez’s) or external timekeepers. The two cycles to be considered are circadian and ultradian.
Psychologists believe that the circadian rhythm (sleep/wake) is controlled by an ep located in the SCN. This clock controls when we sleep and feel drowsy and when we awake and feel alert. It makes sure that our biological processes such as blood sugar and heart rate peak in the day when we need energy and fip at night to save energy. it also makes sure they are synchronised. As this cycle is approximately 25 hours, we need light to act as an ez to help us reset out clock each day to match the earths 24 hours pattern.
Evidence to support this comes from Aschoff. He did an experiment where participants (pps) were put in an underground world war 2 bunker for three to four weeks at a time with no ez’s such as our clocks or light. They settled into a regular 25 hour cycle of sleep wake. This is clear evidence that in the absence of ez’s we have a regular sleep/wake pattern that we must be controlled by a clock. As the pps were out of sync with the 24 hour day, it is evidence that we must use ez’s such as light to re-set our clock.
More evidence comes from Siffre. He lived in a cave underground in Texas for 6 months and was monitored by Nasa. He settled into a 24.5 hour regular cycle but was out of sync with the 24 hour pattern of day and night. Siffre must have been controlled by an ep that was out of sync. He had no light to re-set his clock and so when he came out far more time had passed than he thought. The only explanation for this is that we have ez’s to re-set our clock.
One last piece of evidence to support comes from Miles. He studied a blind man who struggled to stay in sync with the 24 hour day. He had to sedatives at nights and stimulants in the day. This shows that light is incredibly important. it is the most important factor in re-setting our clocks. As the blind man had no light he was unable to stay in sync.
The Ultradian rhythm (stages of sleep) is also controlled by an ep. The ep moves us through the stages of sleep in a regular pattern or cycle. We move through 1, 2, 3 and 4, back up to three and two and then into REM sleep. Each cycle is timed by the ep and lasts 90 minutes. The ep moves us through them, gives us 5 cycles a night and wakes us at the end of the 5th cycle. There are two main ez’s, intense exercise influences the clocks to give us more stage 4 sleep and problem solving in the day influences the clocks to give us more REM sleep.
Evidence to support this comes from Shapiro. He studied ultra marathon runners who had run 52 miles and found that they spend more time in REM sleep.
This is clear evidence that intense exercise is acting as an ez and is influencing the ez to give us more REM sleep.
More evidence to support comes Dement and Kleitman. They conducted sleep lab experiments in which pps were hooked up to an eeg to measure their brain waves. They found that brain waves change throughout the night in a regular pattern as we move in and out of REM. This clearly shows that sleep in as active process, as we are unconscious must be controlled by an ep.
However, the ep’s and ez’s in both the circadian and ultradian rhythm have been studied using only a few participants and quite often in lab conditions. This means that the influence of both the ep’s and ez’s might only be limited to sleep labs where conditions are artificial or to the pps who took part.