Attitude check?

Okay, so I have been confronted lately that I always appear to be upset. I don't mean to have that appearance but I guess it kind of just happens. My dad says that if I don't smile when I'm just looking at someone that it looks like I'm angry. Wow, I can't believe that I have been going around with people thinking that I'm mad. I know sometimes that I am in a bad mood because I don't ever get enough sleep since I work midnight shift and I'm edgy. I get irritated easily but don't mean to. I am not happy about finding out that my face looks and appears upset but I guess now that I am aware of this I can fix the problem. So to everyone who reads this that has had an encounter with me thinking I was in a bad mood, or that I was upset at them, I am so sorry and didn't purpose to make you feel bad or uncomfortable.