Gene differences modify Aschoff's rule in mice.

Nocturnal animals typically display an increase in the free running period of circadian rhythms in response to light in direct proportion to intensity. Here we show that gene differences among inbred strains of mice (Mus musculus) modify the effect of constant bright light on the free running period of a circadian rhythm for wheel running activity. Individuals with the shortest free running periods in dim red light showed the greatest increases in period following exposure to bright light. These effects may be due to gene-imposed differences in the response of a circadian pacemaker to light, or they may be due to gene-imposed differences in visual system function that alter perception of light intensity.