I went in the middle of summer a few years ago with just shorts/t-shirt and
froze my butt off at the top until the sun rose. I actually had a sweatshirt and ditched it in the bushes half way up because it was too hot and I didn't want to carry it. However, once you make it to the very top and stop moving, it is much colder than you would expect and I wished I had warmer clothes until the sun had risen and started warming things up a bit. Everyone up there was huddling close to each other to try to stay warm.
My suggestion would be to, at minimum, throw a sweatshirt or light jacket in your backpack for when you make it to the top and are waiting for the sun to rise..