Voluntary exposure to light by young and aged albino and pigmented inbred mice as a function of light intensity.

Young (5 month) and old mice (23 and 26 months) of the inbred A/J and C5BL/6J strains (N = 80) were isolated in ventilation chambers with ad lib food and water for 1 week at a time. Each animal was allowed to select its own lighting schedule by pressing one lever to turn on a light of either 10.76 or 166.40 lux illumination and pressing another lever to turn the light off. Three-way analyses of variance for Age, Strain, and Illumination effects on parameters of bar pressing, stimulus change, and light duration patterns per circadian period (24 hr) indicate that old mice are less active than young mice and the A/J mice are less active than C57BL/6J mice. Young mice spent more time in the dark than old mice and the A/J albino mice spent more time in the dark than did the C57BL/6J pigmented mice. Other age and strain differences were noted as well and are discussed with respect to some theories of light exposure by nocturnal animals.