The measurement of the diurnal variation of intraocular pressure (IOP) is indispensable for the diagnosis of normal-tension glaucoma (NTG). To determine the diurnal variation of IOP, its measurement has to be made repeatedly for 24 hours, which interferes with patients's sleep at night and may influence the physiologic IOP variation. The authors studied the IOP variation in 82 NTG suspects, whose IOP was first measured every 2 hours for 24 hours. The following night they were suddenly aroused without any notice and IOP was measured. The diurnal IOP variation of NTG patients was found to be similar to that of the normal population and there was no significant difference in the IOP values at the same time points on the two successive nights. Sleep may have little, if any, influence on diurnal IOP variation in NTG patients.