How are you measuring the intensity of your mountain bike rides? Based on those measurements, how intense are they? And do you work better in the morning or the afternoon?
Intensity, duration, and your circadian rhythm could all be factors.
If your mountain bike rides are unmeasured (with either a power meter or heart rate monitor), then they are almost certainly at high intensity. Even measured, it’s damn hard to have a mellow mountain bike ride unless the climbs are super mellow, and the rider, super conservative. If they’re long, intense sessions, then yes, they could definitely be affecting your energy levels.
On the other hand, if they are truly easy sessions (at 75% or less of maximum), then perhaps it’s the timing of your workouts and course work that’s the issue. It’s pretty individual, but I find that I have to do anything creative or mentally taxing in the morning. If I wait until the afternoon, it takes me twice as long and I get half the quality.
The hard part is that I also like to have my most important workout in the morning for the same reasons. So I end up splitting my morning between brain and body time. I leave afternoons for auto-pilot tasks like errands, email, forum posts, and meetings. I know others reverse it, even doing their best work late at night.
I hope that helps.