Why doesnt Odysseus stab polyphemus the first night while the cyclops sleeps?

Odysseus doesn't stab Polyphemus the first night because he realizes that if he kills Polyphemus while he sleeps, they wouldn't be able to move the huge boulder blocking the cave's entrance. Only Polyphemus is strong enough to move it, and without him, the men would be trapped inside the cave. He needs Polyphemus alive in order to escape.