I fell asleep on the Metro the other day. I don’t recommend that you do this. But I was extremely tired.
And, nothing happened to me. I awoke with all my possessions intact and no one had decided to give me a black eye during my slumbers.
So therefore, we can state that, having fallen asleep on the metro, you will not be attacked 100% of the time.
But just in case the probability is, say, 80%, or perhaps 90% — 99%? — I would not make it a regular occurrence.