Previous research on glaucoma medications has been limited to the effects of ocular hypotensive drugs on 24-hour IOP or daytime aqueous humor dynamics. Few studies have evaluated nocturnal aqueous humor dynamics. The investigators recently completed studies of day and night differences in aqueous humor dynamics in patients treated with drugs from three different classes that include a prostaglandin analog, a beta blocker and a carbonic anhydrase inhibitor. The current study is designed to elucidate the physiological mechanisms driving the efficacy of brimonidine, an alpha 2 adrenergic agonist, throughout the 24-hour period, i.e. circadian rhythms in aqueous humor dynamics. Based on what the investigators know of 24 hour IOPs this drug is expected to work well at night potentially by enhancing uveoscleral outflow. This study will test this hypothesis.