He goes back and finds the three asleep and asks them why they could not even stay awake an hour and tells them to pray to avoid "temptation". Temptation might be tempted to fall asleep. In Matthew 6:13 during the Sermon on the Mount he speaks of the temptation of evil, which might be what he is referring to here. He goes back and asks God the same thing, then returns to find them asleep again. They wake up, say nothing, and Jesus leaves for a third time and returns and tells them to get up because the "hour has come" for him to be betrayed.