Maybe this isn't your case, but it COULD be: Are you sure your EYES just aren't "tired"? And I don't even mean they are tired, only irritated from something making it SEEM like you are sleepy/tired? I had gotten new mascara and then kept thinking I was so "sleepy" etc, then I thought, "Hmmm, let me wash my face and take that mascara off!" And viola! I "woke up" and was fine after that!
Also stress and anxiety can cause it - other times I feel so tired and not wanting to do anything - I'd take a Xanax and perk up to my old self. Tiredness GONE (and Xanax is supposed to MAKE you tired!)