Objective To explore the disruption of circadian rhythm induced by the alteration of il?lumination leads to cognition impairement and mood dysreguation in mice. Methods 36 male C57BL/6 mice were divided into constant light group( CL) ,normal light group( N) and constant darkness group( CD) . Open field test was applied for the comparison of locomotor activity,tail suspension test and forced swimming test were conducted for assessment of mood state,and elevated plus?maze and Morris water maze were con?ducted for assessment of cognitive function. Results ( 1) Different circadian rhythms did not change the lo?comotor activity among three groups (CL:(200 160.00±955.28)cm,N:(208 148.00±578.11)cm,CD:(179 128.00±1 185.80)cm, P>0.05). (2) Compared with the control group,CL and CD mice showed significantly decreased immobility time in both TST (CD:(40.16±3.82)s,N:(18.83±2.27)s,CL:(46.00±2.80)s, P<0.01) and FST(CD:(181.33±9.03)s,N:(118.83±7.68)s,CL:(151.83±3.06)s, P<0.05). (3) Compared with the control group,CL and CD mice spent less time (CD:(21.76±6.88)s,N:(80.67±11.19)s,CL:(12.50±5.23)s, P<0.05) and made fewer entries (CD:3.33±0.49,N:6.83±0.91,CL:2.00±0.77, P<0.05) into open arms in elevated plus?maze, and exhibited less crossings in target quarter in Morris water maze (CL:2.67±0.76,N:5.00±0.26,CD:2.83±0.40, P<0.05). Conclusion Chronic constant light or darkness leads to negative impacts on mood and cognition in mice.