http://toxnet.nlm.nih.gov/newtoxnet/lactmed.htm. You can type in a medication to search and it will give you many details including, but not limited to, drug levels, effects in breastfed infants, and so on. I found this tool to be VERY useful and informative. If you ever wonder if a medication is okay to take during lactation, definitely go to http://toxnet.nlm.nih.gov/newtoxnet/lactmed.htm for a sure, supported, and detailed answer.
Today, I searched diphenhydramine. This is the Kroger Nighttime Sleep Aid you see above. After gathering information from the site about the medication, I've decided that I will be taking a dose tonight, but that I will not be making a habit of it.
Ultimately, the decision is up to you and it is you that needs to decide if the benefits outweigh the risks and make the final call. There is no black and white answer.