Hello prefabsprout: You don't say how old you are so I don't know if that could be a factor. I'm an older person & my exercise consists primarily of walking, which I do a lot. I'm no longer on antidepressants. When I was, they always made me more easily tired. But from what you wrote this is apparently not a problem for you.

I walk outdoors in all kinds of weather. And I live in the upper Midwest (U.S.A.) where winters can be tough. Walking in the cold, in snow, on ice & in strong winds tires me out noticeably. My BMI is also at the low end of the healthy range. I have been heavier in years past. I watch my diet carefully in order to keep my weight down where it is today. I have wondered sometimes if I perhaps should be eating more than I do. But I choose not to. I'm almost always hungry. But I have trained myself to ignore this for the most part.

Two other things occur to me. One relates to sleep. For a variety of reasons I have quite a bit of difficulty sleeping. And I believe this contributes to the high level of fatigue I experience pretty-much every day. There was an interesting article regarding sleep in our local newspaper recently. Essentially what it was saying is it's not enough just to sleep. What is important is that a person get sufficient amounts of "quality" sleep... in other words deep levels of sleep because it is during these periods of deep sleep that the body repairs & restores itself. So a person can be sleeping a lot & still not getting the kind of sleep that will allow the body to heal.

The other thing I wonder about is if it is possible you're exercising the same muscle groups too often. I'm no expert, but as I understand it, exercise tears up the muscles & it is the healing that takes place following the exercise that hardens them. (Here again, much of this takes place during periods of deep sleep.) This healing process can take a couple of days or so. So if you're working the same muscles every day, day-after-day, perhaps you're not giving those muscles time to heal. This is where cross-training can be beneficial since different types of exercises exercise different muscle groups.

Anyway, these are the sorts of things that come to mind. Good luck!