ALTERATIONS TO THE RHYTHM
As discussed in previous chapters, it is possible to slow down or speed up a freerunning rhythm in order to resynchronise it back to 24 hours by using Zeitgebers. However other changes to the rhythm are possible.
This is possibly the most dramatic of the change to a circadian rhythm. Arrhythmia is the loss or apparent loss of a rhythm. With arrhythmia, there are two possible explanations. It may be that the clock has actually stopped. Alternatively it may just be that the rhythm is still running, but that its effect is masked.
One of the best examples of arrhythmia is with house sparrows kept in constant bright light. In these conditions, the perching activity of these birds becomes arrhythmic with the sparrows perch hopping 24 hours a day. They apparently lose their alpha/ rho rhythm. Unfortunately it is not possible to separate direct and circadian effects so that it is not possible to tell whether the rhythm is truly arrhythmic or if it is just masked (Binkley 1997)).
If these sparrows are transferred to constant dark from the constant bright light, they become inactive, as they would do if it were night. Activity resumes between 12 and 15 hours after being transferred from constant light to constant dark. This suggests that the clock stopped at end the subjective day when in constant light, at a time when the birds expected it to be dusk. It however may be that the clocks were present but masked, and that transferring the sparrows to constant dark merely reset the clock so that it appears all of the sparrows subjected to this regime appeared to have the same time setting. This resetting using constant light followed by constant dark also has a similar effect on flies, so that it is possible to synchronise the rhythms of populations of flies (Binkley 1997).
Sparrows and flies are not the only organisms to suffer problems when placed in constant light. Evening primroses do not flower and certain forms of algae lose chlorophyll. In many cases the harmful effects of constant light on plants can be countered by using a daily temperature cycle, an example of using multiple Zeitgebers. In many more cases, constant light causes organisms to lose their rhythms slowly, a process called fade out. Constant light does not always produce arrhythmia. It may be that the amplitude of the rhythm is just damped down (Bünning 1973).
When hamster, rats and tree shrews are placed in constant light, the overall period of their cycle increases. However their alpha (activity) time is compressed. In many cases the circadian rhythm will split into two freerunning components, each with a period of approximately 12 hours. This phenomenon is called splitting. Returning the animal to constant dark can return Split rhythms to a single cycle. When this is done, a single freerunning cycle is established within 3-4 days, often with a longer period than for the dissociated rhythms (Earnest and Turek 1982).
It was thought that that splitting is due to the actions of two oscillators, a morning (M) oscillator and evening (E) oscillator. It is thought more likely now that splitting is caused by uncoupling of oscillators located outside the SCN in mammals (Abe et al 2001).
When human rhythms freerun, the body temperature cycle has a separate period length from the freerunning sleep-wake cycle. It is likely that the temperature cycle has a different oscillator to the sleep-wake cycle. Under normal circumstances, the rhythms both have the same period deriving either form a common pacemaker, or deriving from each other. In these circumstances the rhythms are said to be coupled. When the rhythms become dissociated they become uncoupled.
Dissociation could be the reason for jet lag cause by phase shifting in humans. After the phase shift, the various rhythms in the human body resynchronise to the pacemaking rhythm at different rates. These rates depend on the strength of coupling to the pacemaker – the stronger the coupling, the quicker the resynchronisation. The various symptoms of jet lag (fatigue, headaches, insomnia etc) may be due to a synchrony between the various normal rhythms in the body.