Sleep troughs as indicators of endogenous and exogenous components of circadian rhythms

The shape of the envelope of apparent circadian rhythms reflects the endogenous oscillatory drive as well as exogenous influences affecting the investigated variable. As long as the two components are in phase, a rather smooth and regular circadian oscillation usually results. Changing the habitual sleep/wake pattern may thus separate the two forces that determine the shape of circadian variations. As demonstrated in three examples, such separation can be revealed by alterations of the shape of the envelope, by phase shifts, and by best-fitting periods (BFT). In a continuous 6-hr split-shift system, the customary shape of significant circadian oscillations of urinary noradrenaline excretion became bimodal, showing two peaks and two troughs within a 24-hr cycle. Deeper sleep troughs at nocturnal off-duty intervals than when asleep during the day indicated a background presence of the original, although weak, endogenous rhythm. The superimposed bimodal and highly significant apparent rhythm (BFT about 14 hr) resulted from the exogenous effects of the split-wake/split-sleep circadian pattern. The effect of slow crossing of eight time zones on urinary potassium excretion was simulated by living for 8 days on either 23-hr (eastward) or 25-hr (westward) days. The rhythmicity of the gradually changing circadian oscillations was ascertained by substituting BFT (higher %R) for fixed 23-25-hr periods in serial section analyses of the results. Increasing lag in the phase shifts indicated a strong endogenous potassium rhythm whose oscillatory expression was only partly affected by exogenous influences of the gradually changing sleep/wake patterns. Continuously recorded rectal temperature visualized the interplay of the endogenous oscillator and of the exogenous effects of sleep/wake habits in experimental work shifts in which sleep was either suddenly delayed or advanced by 7 hr. The degree of persistence of the endogenous rhythm was especially apparent in the deformations of the shape of sleep temperatures.