OTC sleeping pills on their own wouldn't be efficient. Even benzos aren't unless you took hundreds. I tried taking 100 once and it just caused me to sleep for 24 hours. Only barbiturates on their own will work. But since you're using them to make you drowsy then they will help get the job done with regards to dulling your senses and helping with SI. I don't know if they will be strong enough for something more time consuming like suffocation, though since you could still have time in a semi-conscious state to tear the bag off while you're gasping for air. They would probably only work with a quick death like jumping. Of course it also depends on how strong they are, how many you take, and whether they just make you drowsy or put you in a deep sleep.