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Glaucoma is a leading cause of blindness, especially in older adults. The term covers a group of eye conditions that are usually caused by abnormally high pressure in the eye’s fluid. This problem, called intraocular pressure, damages the optic nerve, which is vital to good vision. The most common form of glaucoma has no warning signs, so it’s important to get regular eye exams that include a test for intraocular pressure. If glaucoma is recognized early, treatment can slow or prevent damage to the optic nerve.

For most people, intraocular pressure naturally changes during the day and overnight. It tends to peak at night because pressure rises when people lie down, and it’s usually higher in the morning than in the afternoon. Because of these fluctuations, a single test during normal office hours may not correctly detect a person’s risk of developing glaucoma or show how their disease is progressing. A test called a diurnal tension curve measures intraocular pressure every few hours over the course of a day.