I have to assume it's from commuting-I never used to have a problem before I started.

I'm getting plenty of sleep at nights, I'm eating better than I was before I started riding so much, but I just get soooo sleepy in the afternoons. Not just, 'oh, i feel a little tired', but 'i want to sneak home and get into bed for 3-4 hrs'. My ride in the morning is only 3 miles, and i've been doing it for about 2 weeks now. I've been riding frequently (avg. 10 miles a day) for about a month. what can I do to shake the sleepies? I don't want to go back to driving a car, but I'm terribly unproductive in the afternoons now.