Could be he isn't getting good sleep. Horses need to be laying down to get REM (deep) sleep. There may be something going on at night that is keeping him alert and on guard.
Both my fillies will fall asleep at the hitching rail and have actually fallen down, saddle and all
ETA- as far as the allergies, Shiloh is on Tri-hist (antihistamine) all summer and I just recently put her on a flax type supplement. It seems to help the ugly itchy's