And try to get more sunlight. If you don't have windows at work, see if you can install full-spectrum bulbs or at least bring in a lamp of your own. It made a huge difference to me when I had an office without windows. If you don't run, try swapping out a brisk 1-2 hour walk for your swim class one or two days a week. Get outside during your breaks, even if it's just to stand in the doorway (trying to avoid the cigarette smokers ). If you're lucky enough to live away from a lot of ambient artificial light, use thin curtains in your bedroom; if you must draw heavy shades at night, then open them in favor of privacy curtains as soon as you're awake in the morning.

But they're right - listen to your body. Once you've maximized your exposure to natural light, fresh air and a good diet, don't try to force yourself beyond that. You are an animal in nature and like it or not, you participate in nature's cycles. IMO, people are healthiest when they acknowledge that and work within it.