Fig. 2. Diurnal intraocular pressure changes. The most common pattern of daily intraocular pressure variation is for the pressure to be highest in the early morning. The magnitude of the variation is usually about 5 mmHg. Considerable individual variability occurs to time of peak and nadir and to magnitude. (Duke-Elder S: System of ophthalmology. Vol 4. The Physiology of the Eye and of Vision. London: Henry Kimpton, 1968.)