Question Are Overnight Buses Comfortable Enough for Sleep?

I'm considering taking an overnight bus for my next trip and I'm curious about the comfort level. I've heard some buses have beds, which sounds like it could be a better option than the standard reclining seats. Does anyone know if these buses are usually well heated or air-conditioned? I'm not expecting luxury, but I do want to feel comfortable, have some privacy, and hopefully get a few hours of sleep. Has anyone had a good or bad experience with overnight buses?