More natural: 1) I'm a little tired today I don't know why.

I'm A little tired today. I do not know what the reason IS.

I'm a little tired today. I do not know why. I do not know the reason. I do not know the reason why. I do not know what the reason is. I do not know what the reason why is. [this doesn't sound too good] "aware" is better when you are told information from someone else. It is probably due to something you did or didn't do.