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In this chapter, travel fatigue and jet lag are treated as a subjective assessment that is synonymous with fatigue and lack of alertness. The symptoms of travel fatigue include a general weariness, and travellers often nap during the journey. The circadian rhythms tend to fall into two groups: those that peak during the daytime and are associated with the active phase of the individual; and those that peak during nocturnal sleep and are associated with fasting and recuperation. The suprachiasmatic nuclei (SCN), paired groups of cells in the base of the hypothalamus, are the site of the body clock in mammals. The body clock transmits its rhythmic output to regions of the brain controlling temperature regulation, hormone release, feeding and sleep. Measured circadian rhythms are combined effects from a body clock (endogenous component) and the environment (exogenous component) that are normally in synchrony.