All works require rest period, interspersed with bouts of work.
Biological Rhythms A complicating factor in all decisions about work and rest cycles are cyclic changes that we all go through in our physiological and psychological states. Figure 2.28 shows some examples of such cyclic changes. Changes Anyone who has flown across the time zones has experienced the effects of that translation in time. For air crew, the Civil Aviation Organization has developed a travel-time formula that combines travel time, the number for time zones crossed, departure time, and arrival time, to specify lengths of rest periods needed to allow at least some adjustment of circadian rhythms. A quarter of the American labor force is engaged in shift work.