Human circadian rhythms in activity, body temperature and other functions.

From earlier studies in the field of 24-hr rhythms, it has been concluded that the rhythm is an exogenous one, governed by social influences on behavior and, perhaps, by cosmic stimuli. Recently, new steps have been made in two directions: 1) the phase-relationships between many rhythmic functions have been described in detail; 2) it has been demonstrated that the rhythm is based on an endogenous, self-sustained oscillation. In conditions of isolation, subjects show a 'circadian' rhythm whose frequency deviates from that of the earth's rotation. In case of such free-running rhythms, it may happen that different functions show different frequencies (internal desynchronization), suggesting that there exists a multiplicity of oscillators in the organism. The implications of these findings for problems in applied physiology are exemplified by the results of experiments in which organisms were exposed to shifts of the entraining light-dark cycle (simulating flights in eastward or westward direction). Re-entrainment lasts longer (and hence also the time of diminished efficiency) after a westward than after an eastward flight.