Exogenous and Endogenous Influences on Rhythms after Sudden Time Shift

When subjects are following their usual nychthemeral habits, the observed rhythms result from the interaction of a circadian ‘clock’ with rhythmic external influences. These major external influences can be removed by spending 24 h under constant conditions. The endogenous rhythms thus revealed, in temperature and urinary excretion, have mostly an earlier phasing than nychthemeral rhythms and adapt more slowly to time shift, usually by a phase delay. It appears undesirable for the rhythms of shift workers to be entrained to time shifts, and means of retaining the usual phasing are discussed.