Effect of Space Flight on Circadian Rhythms

Abstract

Neuroscience Institute, Morehouse School of Medicine, Atlanta, GA, USAIntroductionDaily rhythmicity is a ubiquitous feature of living systems. Generally, theserhythms are not just passive consequences of cyclic fluctuations in the environ-ment, but, rather they originate within the organisms. They are generatedand controlled by endogenous physiological oscillators. The fundamentaladaptive function of an endogenously programmed rhythmicity is to providean optimal and anticipatory temporal organization of physiological processesand behavior in relation to the environment. Endogenously controlled dailyrhythms are called circadian rhythms, referring to the fact that they have aperiod of about a day. Under conditions of temporal isolation this period,in most cases, slightly deviates from exactly 24 h. Synchronization of endo-genous circadian rhythms to the environment occurs through daily adjustmentof the clock by external time cues or zeitgebers. The most important and reliablecue for synchronization is the daily light–dark cycle (Pittendrigh and Daan,1976).Properties of circadian rhythmsSince some of the terminology used in this review is specific to the circadianspecialty, we will begin this chapter with an explanation of the most commonterms used to describe circadian rhythms. A circadian clock is a part of anorganism which is capable of generating a self-sustained rhythm with a periodclose to 24 h. A rhythm is characterized by three features: period, amplitudeand phase (Fig. 1). The period is the length of time necessary to completeone full cycle, the amplitude is, roughly, the difference between the maximumand the minimum levels observed during a full cycle, and the phase refers to thetemporal relationship between a specific identifiable point on the cycle and apoint on a reference cycle.When an organism is maintained under constant conditions its circadianrhythms persist with a period close to, but different from 24 h. The fact thatrhythmicity persists under constant condition demonstrates that the rhythmis endogenous. The period of a rhythm under constant conditions is called its