LIGHT SUPPRESSES FREQUENCY AND ENDOGENOUS AMPLITUDE OF THE CIRCADIAN SYSTEM IN NOCTURNAL ANIMALS

According to Aschoff's rule the period of free-running circadian rhyth ms of nocturnal animals lengthens with increasing constant ambient lig ht intensity. The present study investigated Aschoff's rule in relatio n to the endogenous circadian temperature rhythm using a recently deve loped mathematical model. Rats were exposed to various ambient light c onditions while their free-running sleep-wake and temperature rhythms were recorded continuously. The results show that both frequency and e ndogenous amplitude were suppressed by light, indicating that circadia n rhythmicity may disappear as a result of a suppression of the endoge nous rhythm at long circadian cycle lengths. Furthermore, different li ght conditions producing significantly different free-running periods were accompanied by virtually identical activity levels. Therefore, it is concluded that the effect of light on the free-running period is t he result of a direct modulation of the circadian system, rather than of indirect action by altering levels of locomotor activity.