Lack of evidence that feedback from lifestyle alters the amplitude of the circadian pacemaker in humans

Two groups of healthy subjects were studied indoors, first while living nor mally for 8 days (control section) and then for 18 x 27h "days" (experiment al section). This schedule forces the endogenous (body clock-driven) and ex ogenous (lifestyle-driven) components of circadian rhythms to run independe ntly. Rectal temperature and wrist movement were measured throughout and us ed as markers of the amplitude of the circadian rhythm, with the rectal tem perature also "purified" by means of the activity record to give informatio n about the endogenous oscillator Results showed that, during the experimen tal days, there were changes in the amplitude of the overt temperature rhyt hm and in the relative amounts of out-of-bed and in-bed activity, both of w hich indicated an interaction between endogenous and exogenous components o f the rhythm. However, the amplitude and the amount of overlap were not sig nificantly different on the control days (when endogenous and exogenous com ponents remained synchronized) and those experimental days when endogenous and exogenous components were only transiently synchronized; also, the ampl itudes of purified temperature rhythms did not change significantly during the experimental days in spite of changes in the relationship between the e ndogenous and exogenous components. Neither result offers support for the v iew that the exogenous rhythm alters the amplitude of oscillation of the en dogenous circadian oscillator in humans.