Persistence of circadian oscillation while locomotor activity and plasma melatonin levels became aperiodic under prolonged continuous light in the rat.

In order to examine the mechanism for a loss of circadian rhythms in several functions under prolonged continuous light (LL), rats were blinded following LL over 5 months, and the mode of reappearance of circadian rhythms were analyzed in locomotor activity and plasma melatonin levels. Locomotor activity and plasma melatonin levels in individual rats became aperiodic after the exposure to LL. On the day of blinding, plasma melatonin levels showed circadian rhythms having a peak coincided with the activity time of locomotor rhythm which was restored after blinding. The time of melatonin peak was not related to the time of blinding (onset of darkness) nor to the initial time of blood sampling. Circadian rhythm in plasma melatonin levels reappeared faster than those in locomotor activity. The findings suggest that aperiodism developed in these functions under prolonged LL is not due to disruption of the circadian oscillation but to uncoupling of overt functions from the circadian pacemaker.