Light suppresses frequency and endogenous amplitude of the circadian system in nocturnal animals

According to Aschoff s rule the period of free‐running circadian rhythms of nocturnal animals lengthens with increasing constant ambient light intensity. The present study investigated Aschoff s rule in relation to the endogenous circadian temperature rhythm using a recently developed mathematical model. Rats were exposed to various ambient light conditions while their free‐running sleep‐wake and temperature rhythms were recorded continuously. The results show that both frequency and endogenous amplitude were suppressed by light, indicating that circadian rhythmicity may disappear as a result of a suppression of the endogenous rhythm at long circadian cycle lengths. Furthermore, different light 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 the result of a direct modulation of the circadian system, rather than of indirect action by altering levels of locomotor activity.