all 'exertional' exercise produces a stress on your body. i'm not talking about just the physical stress, but the mental stress that comes with a release of 'stress hormones' these 'stressors' act in a sequence that mimics a cascade that then effects other hormones that regulate everything from serotonin (sleep) to specific hormones that guide metabolism, mood, etc.
if you are a cyclist that pushes oneself to get better due to a training situation, that is very different than tooling around.
that being said, i find that i do my best biking (and most enjoyable biking) when i am in a rythmic zone, weather i am combating a headwind or not. in a safe ~65-70% 'recovery' heart rate, a trained athlete can and should feel the calm - as evidenced by the low resting heart rate and the resulting 'zone' that you can get in.
i find a parallel between the mental benefits gotten from my yoga classes and my biking. i have biked 70-90 miles and been so focused on nothing much of anything and suddenly find myself at the end of my ride.
i'd look into the EEG results of wind tunnel tests done with the olympic training in colorado. there must be EEG results on file with those athletes, no?
what about articles written by RAAM athletes?