On the past two long rides, both 200+ milers I've had the same experience that has been really strange especially given the time of day.
Since I'm averaging a 17 mph pace I'm doing the ride in one day. I've been getting plenty of sleep overnight but come around 12-2PM, I all of a sudden find myself getting sleepy. It's been rather unnerving, especially back on Monday. I found my eyes really wanting to close on one me. The temps have generally only been in the 70s when I've been doing the rides. Both times toward the end of the ride I have popped out of it a couple of hours before I get home. In fact on Monday at the end of the 225 miles I felt like I could do 300 without trying. I just would have needed a headlight. Granted my knees were starting to feel it and so were the thighs. I pressed way too hard in the beginning of the ride and I was paying for it quite nicely.
The only thing I can think of that might cause an effect like it would be digestion. Under the circumstances I wouldn't think that would be causing the problem so I'm just trying to see if any of you have had the experience and what else might be causing it.