ds2 would only sleep this way too, i was nervous to leave him like that, so instead i would put him in the bassinette in between 2 rolled up towels, on his side, his head was out the top of the towels so no way he could get smothered, he was in so tight he couldn't go on his tummy, he was fine like that. He's still a tummy sleeper now. I think if you check on him often, or listen for his breathing on the monitor its fine.