Over-the-counter sleep aids can be effective for an occasional sleepless night. There are a few caveats, however. Most over-the-counter sleep aids contain antihistamines. Tolerance to the sedative effects of antihistamines can develop quickly — so the longer you take them, the less likely they are to make you sleepy.
In addition, some over-the-counter sleep aids can leave you feeling groggy and unwell the next day. This is the so-called hangover effect. Medication interactions are possible as well,and much remains unknown about the safety and effectiveness of over-the-counter sleep aids.