Intraocular pressure (IOP) varies dynamically throughout the circadian cycle. IOP elevations during the nocturnal period may be particularly important in the pathogenesis of glaucoma, although sleeping IOP cannot be measured at this time. Additionally, IOP fluctuations may be an independent risk factor for glaucoma. However, not all glaucoma therapies are equally effective at lowering IOP throughout the 24-hour period. The prostaglandin analogs have excellent IOP control throughout the 24-hour period, although less at night than during the day. In contrast, some other classes of medications, such as the beta-blockers, have little or no IOP-lowering effect at night. The prostaglandin analogs also have excellent persistency of IOP lowering, lasting at least as long as the 24-hour dosing period, and likely much longer. Glaucoma filtering surgery appears to have even better 24-hour IOP reduction and smaller fluctuations than maximal medical therapy including prostaglandin analogs.