(A) SST-/- mice displayed hyperactivity during the second half of their active phase as indicated by total distance walked in the open field, compared to their SST+/+ littermates. (B) However, time spent in center did not differ between genotypes are time of testing. (C,D) In the light/dark- avoidance test, SST-/- mice showed reduced exploration of the light compartment, indicating increased anxiety-like behavior during the second half of the active phase. In fact the mutants failed to show a reduction of anxiety as seen in SST+/+ mice, thus indicating a deficit in the circadian modulation of anxiety- like behavior. Values are mean ± SEM. * significant differences between time point of testing, p<0.05; #, significant differences between SST-/- and SST+/+ mice, p < 0.05; # # p < 0.01.