After I started going (most times during 30mins before I lunch) to gym and do cardio as well as weights, I do not get such effects anymore. Basically you are fitter so can "dong" better, also your basic metabolic rate is much higher during rest as your body is trying to repair muscles and stuff, so it does not dip too much and make you sleepy.
Actually a prolonged sedentary lifestyle is the base of many health problems later when you grow old. So do start exercising.
It also has the added benefit of aiding you lift your tele-lenses for long periods. Just last 2 weeks I was shooting those Chinese New Year fashion/costume/cultural shows at Raffles City and I did not have any problems at all even for a 40 min show with a body, flash and 70-200/2.8 lens.
Just think, if you are fitter and can lift and endure weights much better, your 1.2kg lens becomes a 400-500g one. Wah, FOC! hehe...