Taking Benadryl occasionally is okay to help you sleep or treating other things such as sneezing, allergies, runny noses etc.
But you should avoid using Benadryl long term or everyday or every night if possible since it can cause other problems.
Some side affects that long term Benadryl use can cause are, Nausea, Anxiety, Fast Heart Rate, Blurred Vision, Dry Mouth, Constipation and Difficulty Urinating.