overheating at night

i have an 1987 cherokee 4.0L that is only overheating at night. it is fine during the day. i just replaced the radiator and lower radiator hose. im pretty sure there's no air in the system. any suggestions why it would only overheat at night??

I'd have the charging system checked by taking the battery and alt. out and taking them to the parts store to have them tested it sounds like while driving it at night with all the lights on the electric fan is not getting the juice it needs.