Outline the main endogenous pacemaker and exogenous zeitgeber in the sleep-wake cycle regarding research, and outline one advantage and one disadvantage of this arrangement.

First, it is important to define the features of the mammalian sleep wake cycle. Sleep-wake comes under the category of a circadian rhythm; this is an approximately 25-hour cycle in the biochemical, physiological or behavioural processed of living organisms, and includes other cycles such as: hormone production, heart rate and blood pressure. It is fundamental that our bodies maintain this ~25-hour cycle, and so endogenous pacemakers come into play. These are pacemakers that belong within the organism rather than the environment and keeps the time of the cycle stable, helping to maintain its function. It would be negative in many respects if our sleep-wake cycle fluctuated from a few hours to a couple of days, and so having a functioning endogenous pacemaker is fundamental. The main pacemaker in mammals is the suprachiasmatic nucleus in the hypothalamus. This idea is supported by evidence from a study by Stephen and Nunez (1977); they created lesions in the suprachiasmatic nucleus in hamsters and found that the sleep-wake cycle was thereafter eliminated. Understandably, having a 25-hour natural sleep-wake cycle raises some issues when we are confined to a 24-hour day. Therefore, we need exogenous zeitgebers to help 'reset' our body clocks. Exogenous means that the zeitgeber (meaning 'time-giver') comes from outside the organism, from the environment. In the sleep-wake cycle the main exogenous zeitgeber is light. The suprachiasmatic nucleus (the main endogenous pacemaker) receives information about light from the optic nerve, even when the eyes are closed. Evidence supporting this theory comes from more experiments on hamsters. When the fibre leading form the optic nerve to the suprachiasmatic nucleus was cut - although they could still see - they could not reset their circadian clocks (Rusak, 1977). The morning light shifts the internal clock forwards or backwards, thus keeping the rhythm in pace with the environment. In addition to this, the pineal gland (at the top of the mid brain) is also sensitive to light. So, when light levels are low, the pineal gland secretes melatonin which makes us sleepy. The opposite happens when there is a lot of light: this prevents melatonin being released which makes us feel awake. Research found they could change people's circadian rhythms by shining a light on the backs of people's knees! The advantage of having both internal and external rhythms is that if we were controlled only by external cues, our biological system would change too much (therefore we have an internal body clock), but we also need to react to changes in the environment (which is why we need external cues). One disadvantage of this arrangement is that the rhythms are sometimes too slow to change, such as with jet lag and shift work, which leads to other problems, such as tiredness.