Yes! I leave my computer on during the night! I leave it on all the time.
Computers suck energy. When you leave for the night you are suppose to turn it off to save. Some even go as far as turning it off when they leave for a couple hours. Some put on a screen saver, but apparently a screen saver uses just as much as the computer would anyways. So turn it off.
Ha, I don't. It takes way too long for my computer to turn on. My laptop, well let's say I'm ready to move on and do something else by the time that it is ready for me to use it. I am not a very patient person. I want my computer to be ready when I get in. Now, if it took less than a minute to boot up, I'd consider turning it off, but alas, it does not, so my computer stays on all the time.