Nail biting is a sign of anxiety. If she only bites her nails at bed time, talk to her and ask her why (if she is old enough to talk). She may be suffering from separation anxiety or perhaps is afraid of the dark? Does she have bad dreams or talk in her sleep? If any of this is a concern mention it to your doctor.
Otherwise, tell her about biting being hurtful, and that she shouldn't bite anything, especially not herself. Kiss her fingers or treat them like any other "boo-boo" after she bites them. If she is just tired and notices her nails being long or something, then make nail clipping and grooming part of her night time bed routine. Cuddle with her on your lap and cut her nails and file them and use lotion on her hands then put her to bed. Lotion usually also tastes bad but its not toxic so that may discourage her from chewing.