ABSTRACT
In Chapters 5 and 6, it was repeatedly emphasized that, under constant environmental conditions, circadian rhythms freerun with periods slightly different from 24.0 hours. The fact that the periods are not exactly 24.0 hours was presented as a benecial feature that allows us to be condent that the periodicity is truly endogenous and not merely induced by undetected environmental cycles. However, what is benecial to the circadian physiologist is not necessarily benecial to the organism. Indeed, the fact that the circadian period is not exactly 24.0 hours means that in the real world-where the Sun rises and sets every 24.0 hours-organisms can get into serious trouble. If a rat’s internal clock tells it that it is midnight when it is actually midday, the rat may venture out of its burrow at the wrong time of day and end up being eaten by a hungry dog. Likewise, if your internal clock tells you that it is 8 a.m. when it is actually 6 p.m., your half-hour drive to work will take you only to a closed ofce. Evidently, the circadian pacemaker must be able to synchronize to the cycle of day and night if the organism is to thrive in the real world.
