I know you feel rested when you wake up, but have you tried sleeping for longer than 6 hours? Some people can function on that little sleep, but a lot of people can't.
You said it doesn't happen on the weekends. Do you sleep more on the weekends? Do you eat differently? Like do you eat cereal during the week and eggs on the weekends? Do you drink more caffeine when you are at home?
Do you like your job? Maybe you are bored? Are the lights at your work station bright? Is there any sunlight from a window? If not, try a bright light at your desk to signal to your body that it's daytime. Do you have a long commute? Have you tried small snacks throughout the day? Maybe it's a blood sugar thing?
Since it doesn't happen on the weekends, it's probably not medical. There is something in your routine or the environment that is making a difference. Try changing one thing that you do on the weekends to the weekday. Try it for a couple of days, if that doesn't work, try something else. That way, you can figure out what it is. If nothing works, try going to the doctor for a check-up and see if maybe it's a blood sugar thing or a hormone imbalance.