A characteristic property of circadian rhythms is their ability to be synchronized, or entrained, by external time cues, such as the light-dark cycle. Although circadian rhythms can persist in the absence of external time cues, meaning that they are not driven by the environment, normally such cues are present and the rhythms are aligned to them.
Accordingly, if a shift in external cues occurs (e.g., following travel across time zones), the rhythms will be aligned to the new cues. This alignment is called entrainment.
Initially, it was unclear whether circadian rhythm entrainment was achieved by modulating the rate of cycling (i.e., whether the cycle was shortened or lengthened until it was aligned to the new cues and then reverted to its original length) or whether entrainment was achieved by discrete “resetting” events. Experiments resulting from this debate led to fundamental discoveries.
For example, researchers discovered that the organism’s response to light (i.e., whether a cycle advances, is delayed, or remains unchanged) differs depending on the phase in the cycle at which it is presented (Pittendrigh 1960). Thus, exposure to light during the early part of the individual’s “normal” dark period generally results in a phase delay, whereas exposure to light during the late part of the individual’s normal dark period generally results in a phase advance.
This difference in responses can be represented by a phase-response curve (see figure 1). Such a curve can predict the manner in which an organism will entrain not only to shifts in the light-dark cycles but also to unusual light cycles, such as non-24-hour cycles or different light:dark ratios. The existence of a phase-response curve also implies that entrainment is achieved by discrete resetting events rather than changes in the rate of cycling.
Figure 1. A representative circadian rhythm is depicted in which the level of a particular measure (e.g., blood hormone levels and activity levels) varies according to time. The difference in the level between peak and trough values is the amplitude of the rhythm. The timing of a reference point in the cycle (e.g., the peak) relative to a fixed event (e.g., beginning of the night phase) is the phase. The time interval between phase reference points (e.g., two peaks) is called the period. The rhythm shown persists even in continuous darkness (i.e., is free running).
Other Circadian Rhythm Entrainment Factors
In addition to the timing of the light exposure, the light intensity can modulate cycling periods when organisms are left in constant light. Thus, exposure to brighter light intensities can lengthen the period in some species and shorten it in other species. This phenomenon has been dubbed “Aschoff’s rule” (Aschoff 1960).
Ultimately, both mechanisms of circadian rhythm entrainment appear to be aspects of the same thing, because the consequences of Aschoff’s rule can be predicted or explained by the phase-response curves to light.
Although the light-dark cycle clearly is the major Zeitgeber for all organisms, other factors-such as social interactions, activity or exercise, and even temperature-also can modulate a cycle’s phase. The influence of temperature on circadian rhythms is particularly interesting in that a change in temperature can affect the phase of a cycle without substantially altering the rate of cycling.
This means that the cycle may start at an earlier or later-than-normal time but still have the same length. On the one hand, this ability of the internal clock’s pacemaker to compensate for changes in temperature is critical to its ability to predict and adapt to environmental changes, because a clock that speeds up and slows down as the temperature changes would not be useful.
On the other hand, temperature compensation also is rather puzzling, because most kinds of biological processes (e.g., biochemical reactions in the body) are accelerated or slowed by temperature changes. Ultimately, this riddle has provided a clue to the nature of the internal clock – that is, the fact that circadian rhythms have a genetic basis.
Such a program of gene expression would be more resistant to temperature alteration than, for example, a simple biochemical reaction.