Like many people have said above, the logical conclusion is you're just not getting enough sleep.
The idea that 8 hours of sleep is ideal doesn't really have any scientific basis at all (penn and teller did an episode on this haha so its what im going off).
Maybe youre one of those people who really need 9 hours or something. Most of us could use more sleep than we get to be honest, people used to sleep way longer before electric lighting.
If you dont have some urgent need to stay up and do work i think getting a decent sleep is a good idea. You concentrate and learn much better all day and i hear it helps with retaining memories. For me personally i also found it impacted my health overall, if i didn't sleep enough i just started getting random headaches or aches and things like that.
If you dont get enough sleep the night before, it puts you out for all of the next day as well, then you need more sleep the next night to catch up.
Try sleeping longer and see if that works, if not maybe see your doctor.
I take heaps of vitamin C everyday... helps get rid of any colds coming on also... don't know if it helps with tiredness though. 
Just on a little side note huge doses of vitamin C have been proven to
not work for preventing the common cold and if its that chewable form for Vitamin C it can really screw up your teeth (Thats the acid bit of the Ascorbic
Acid)