The intraocular pressure in most eyes follows a diurnal rhythm characterized by a morning rise and a subsequent falloff through the afternoon. The effect of this diurnal pattern was assessed at a glaucoma detection center by randomly assigning subjects to morning or afternoon screening clinics and comparing the percentages screened positive at both times of the day. Utilization of the same screening cutoff level in the morning and afternoon clinics yielded a higher percentage of subjects screened positive in the morning. In addition, both morning and afternoon tonometric measurements were obtained in a large number of individuals, and the majority of their pressure recordings were lower in the afternoon. The findings indicated that a large proportion of positively screened eyes in the morning would have been missed in an afternoon detection clinic because of a falloff in pressure towards the later part of the day.
Lennon RG, Turnbull CD. Diurnal Intraocular Pressure Variation in a Glaucoma Screening Program. Arch Ophthalmol. 1968;80(6):714-717. doi:10.1001/archopht.1968.00980050716006