I get that sometimes, and as 'genec' said, it seems like it happens before i get a cold or something. coincidence maybe? i'm not sure. But it's similar to a runner's high, and usually happens when i've taken a break, maybe 10 minutes or so (I.e running into my LBS) and when i get back on the bike feels lighter and just easier to pedal. It's not the same as a runner's high, because when I get that it's more trance-like than what happens biking.