I think its pretty well documented that adding regular exercise to a sedentary lifestyle should help you to feel less lethargic. If you're already exercising regularly, then I'd agree that upping your exercise wouldn't do much for you.
I definitely agree on the sleep- I do best on 8-9 hours. You might want to keep a journal on when you feel the most fatigued, which may help point you to the cause. You might be looking at a blood sugar crash, you might be lacking in sleep, or if it's all the time, it might be a nutritional deficiency. Don't count out mental exhaustion, either- if you're overly stressed, some stress reduction or relaxation techniques might help.
Another suggestion would be to try exercising anyway- tell yourself you only have to do something for 10 minutes, then if you're still tired you can stop. Most of the time, though, you'll keep at it once you get started. I'm not talking an all-out, push yourself to the point of collapse workout- just a moderate intensity, get the blood pumping workout. Often, by the time I get done with one of these, I'll find I have energy to do other stuff around the house that I wouldn't have done if I had just come home and sat on the couch and I'll actually sleep better that night too. If you have trouble getting up out of your chair, then don't sit down to begin with. When I go to the gym after work, my rule is no sitting down except to put on my shoes. Walk in the door, change clothes, grab the water bottle and go!