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Full list of author information is available at the end of the articleBackground Open-angle glaucoma is a chronic neurodegenerative disease with progressive loss of retinal ganglion cells and optic nerve fibers accompanied by characteristic cupping of the optic nerve head. In clinical practice, intraocular pressure (IOP) is the most important risk factor for glau-coma because lowering of IOP is the only therapeutic option in glaucoma whose effectiveness has been proven in large randomized clinical trials [1-3]. IOP fluctuates rhythmically with a 24-hour period (diurnal variation), but it also shows rhythmical fluctua-tions with other phase lengths and sporadic fluctuation [4]. Diurnal variation of IOP has been examined in clinical studies for a long time, because it was felt that “it concerns itself with one of the most fundamental aspects of the disease ” [5]. In eyes with glaucoma, timing of IOP vari-ation was believed to be generally more erratic [6]. Newer studies support this notion, and it has been demonstrated that the responsible circadian timing system can be altered in glaucoma [7]. Furthermore, a larger magnitude of IOP fluctuations is generally considered characteristic o