Npas2 mutant mice respond more on an active lever for cocaine, particularly females in the dark phase. A, At ZT2, during the light phase, increased active lever pressing for cocaine is seen in (A) female and (B) male Npas2 mutant mice. C, Although this effect seems to be greater in female mice, no sex difference was found. All Npas2 mutant mice pressed the active lever more than WT mice (no post hocs shown), while only a trending increase in inactive lever pressing was found. Both WT and Npas2 mutants press the active lever more than the inactive lever. D, In the dark, or active, phase (ZT14), active lever pressing is elevated in female Npas2 mutant mice (no post hocs shown), (E) but not male mutants. Both WT and Npas2 mutants press the active more than the inactive lever. Mean + SEM; n = 10–19.