When your tired you are slower, focus and concentrate less, so therefore you won't be so worried/care so much about the smaller things that people could catch you out on.
In the day you are sharp and focused, and will be worried and nervous that you might screw up a tiny detail that most of the time won't matter, this is good because it keeps you on your toes and you will make less mistakes.
I use to be the same way, it was only at the start though the more that I got into it the less I worried. So my advice to you is perfrom for a friend or something first where you know that you can mess up. Once you start to perform everything else just seems to just fall into place.