The influence of fatigue and rest period on the circadian variation of error frequency in shift workers (engine drivers)

The mechanism underlying the endogenous diurnal periodicity of biological processes can be considered a self-sustained oscillation, which can be entrained to an external cycle. In such oscillations the phase-angle of the entrained cycle depends upon the spontaneous frequency (free-running period) of the oscillator. The activity rhythm of lizards kept in constant light, and in a sinusoidal 24-hour temperature cycle, showed entrainment to this cycle. The phase of the entrained rhythm depended on the spontaneous frequency which was expressed in constant conditions occurring immediately before or after the exposure to the extraneous cycle. This is the first experimental demonstration showing the dependence of phase on the free-running period in an endogenous diurnal rhythm.

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.