Nature of the light stimulus producing Aschoff's intensity effect and anovulation.

Using feedback circuits, light exposure was linked to wheel-running activity in female albino rats. Because the photosensitive portions of the circadian cycle are known to coincide with wheel-running activity, the feedback circuits concentrated light on the photosensitive portions of the cycle. In this type of lighting, the free-running period of locomotor activity was directly proportional to the light intensity (i.e., the Aschoff effect), and at an intensity of 100 1x, cyclic ovulation caused. Both these effects, which were previously thought to result only from exposure to continuous light (LL), occurred even though these rats were exposed to only 4 h of light per circadian cycle. These results indicate that the consequences of LL are not due to the continuity of the light per se but represent the effects of light falling on discrete photosensitive portions of the circadian cycle.