<i>Per1</i> mutation enhances masking responses in mice.

Light can restrict the activity of an animal to a diurnal or nocturnal niche by synchronizing its endogenous clock (entrainment) which controls the sleep wake cycle. Light can also directly change an animal's activity level (masking). In mice, high illumination levels decrease activity, i.e. negative masking occurs. To investigate the role of core circadian clock genes <i>Per1</i> and <i>Per2</i> in masking, we used a 5-day behavioral masking protocol consisting of 3 h pulses of light given in the night at various illuminances (4-5 lux, 20 lux and 200 lux). Mice lacking the <i>Per1</i> gene had decreased locomotion in the presence of a light pulse compared to wild-type, <i>Per2</i> and <i>Per1 Per2</i> double mutant mice. <i>Per2</i> single mutant and <i>Per1 Per2</i> double mutant mice did not show significantly different masking responses compared to wild-type controls. This suggests that <i>Per1</i> suppresses negative masking responses in mice.