Can i keep nix (permethrin) lice treatment on overnight?

No: The manufacturer recommends to leave it on the patient's hair for 10 minutes. Most importantly, this is an insecticide that paralyze the lice, thus what you want is to grab the fine comb and do an extensive combing as indicated in the instructions, so the patient does not shed the lice all over. Keep in mind that there is resistance to this type of drugs, thus follow up with pcp is important.