While there are many sleep aides that have the potential to be addictive, none of them can be purchased over the counter. What you likely bought was some formulation of diphenhydramine (the active ingredient in benadryl). While this is not habit forming, it is probably not a good idea to take every evening. In addition, the sleep aides will make it more difficult to wake up when your newborn needs attention (as they often do multiple times during the night).
I should note that the sleep aides available today that are considered somewhat habit forming have a fairly low risk of being a problem. I think their may be better options for you if you decide to speak to your doctor
about this issue. I suggest that you schedule an appointment with your primary care physician
. The two of you can discuss your current sleep schedule an decide if their is a better plan available for you to get more sleep. This way you can have a professional perspective to this issue. It may be that your doctor is fine with your taking the over the counter medication, but you should certainly have him or her on board with that plan. Good luck.