Abstract Purpose: To characterize the circadian variations of intraocular pressure (IOP) in the normal tension glaucoma (NTG). Methods: Fourteen patients with NTG were studied. The age range was from 45 to 87 years old. All patients underwent a complete ophthalmologic examination (including corneal thickness) and visual field (Humphrey type). Then, IOP measurements were hourly controlled by Tonopen TM*, during a 24 hour cycle associated with sleep monitoring. The sleep study included a polysomnography (to exclude a sleep apnoea syndrome) and was associated with 24 hour measurements of blood pressure and heart rate. The nycthemeral rhythm was defined according to the cosinor technique. Results: Sixty percent of the patients exhibited an absence of circadian rhythm of IOP among 24 hours. Twenty one percent of the patients had a nocturnal rise of IOP and 21% of the patients had a diurnal rise of IOP (< 21 mmHg). A sleep apnoea syndrome was diagnosed in 71% of the patients, abnormal nocturnal dips of arterial pressure were present in 78%, a vasospastic syndrome was present in 41.6% with and 40% of the patients exhibited an systemic hypertension. Conclusions: In our study, most of the patients shown an absence of nycthemeral rhythm of IOP. In this population, a high prevalence of sleep apnoea syndrome, of abnormal nocturnal dips of blood pressure, and of vasospastic syndrome was found.