I don't know... don't you feel a bit knackered after flying, though this is illogical after you've been sat still in a seat for so many hours? Perhaps it's because the adrenaline of a holiday or a weekend break knocks off during your time on the plane and it's back to reality, or a message to remind you of your overindulgence?
Anyway, what distance are we talking about from gate to station (never flown from Luton, nothing personal, just never needed to?)