SUMMARY
The arteritic and non-arteritic types of the anterior ischemic optic neuropathy (A-AION and NA-AION) were studied in 168 patients concerning the time of onset of the visual loss. Thirty per cent of the cases of A-AION and 64% of the cases of NA-AION occured during the night and were therefore observed on waking in the morning. Seventy-three per cent of patients with idiopathic NA-AION and 44% of patients with NA-AION due to identified disorders developed optic nerve ischemia when sleeping in the night. There was a statistically significant predominance of onset of idiopathic NA-AION in the night. This fact may be related to the fall of the perfusion pressure in the night, associated with structural changes of the optic disc which turn it more susceptible to ischemia.