Answer: First this information is provided for educational purposes only - this is not intended to provide an exhaustive explanation or any guide to treatment. If you are contemplating suicide, please seek expert medical advice or contact a suicide hot-line (1-800-SUICIDE or 1-800-784-2433 or http://suicidehotlines.com). Do not try to "go it alone" they can help. Most people who survive suicide attempts later regret their attempts.Most commonly available sleep aids (over the counter) are simply the antihistime benadryl (diphenhydramine). They are not as dangerous as prescription sleep aids (hence its availability OTC). Overdose signs and symptoms can be found it the reference below. Some drugs like tylenol pm have (obviously) tylenol which can cause liver failure and death in larger doses.There are two major classes of prescription sleep aids, the benzodiazepams (BZDs) and the non-benzo's.BZD can kill by supressing the natural respiratory drive and inducing a stupor. They can be treated with a competitive antogonist (a drug that competes for the targets binding site, but does not actually cause an effect) called flumazinil.The newer classes are not BZD based, but many are still classified as sedatives. They do not appear as dangerous as BZDs, but can still be lethal. Treatment is likely supportive.Regardless of the specific drug used, many people die because they go into a light to deep coma and can no longer protect their airway - this can result in the tongue occluding the airway (they suffocate) or the vomit and aspirate (draw the vomittus into the airway) and die of chemical pneumonia days to weeks after they are treated for the acute overdose. There is also the possibility of associated trauma due to such things as operativing a motor vehicle while under the influence of such drugs (both unwise and illegal)