REM phase: An ingenious mechanism to enhance clearance of metabolic waste from the retina

The Rapid Eye Movement (REM) phase of sleep, also known as "active sleep" because of physiological similarities to waking state, is characterized by intense cerebral electrical activity, propensity to dream vividly and suppression of skeletal muscle activity (atonia) except for the extraocular muscles which give rise to the so-called REM. In 1998 David Maurice, an ophthalmologist, proposed that REM sleep was associated with an eye function: it would be required to stir the anterior chamber and bath it with aqueous humor to prevent corneal anoxia during sleep. However, potential metabolic problems could arise in the outer retinal layers which lack a direct blood supply. New research lends support to the hypothesis that a para-vascular transport system, the so-called "glymphatic", is present in the eye analogous to the one recently discovered in the brain. It is a functional waste clearance pathway which promotes elimination of interstitial solutes from the brain along para-vascular channels. Glymphatic function increases during sleep and just as a "brain pump" moves fluids in the central nervous system, a "vitreous pump" moves them into the eyeballs during REM phase. A number of similarities between Alzheimer's disease and several retinal degenerations have been described, particularly with respect to either age-related macular degeneration and chronic open-angle glaucoma. Impairment of this mechanism in some disease states and in the normal aging process could have serious consequences for visual function. In this manuscript I propose a new hypothesis regarding the role of REM phase on physio-pathology of the human eye: it would be an ingenious mechanism to enhanced clearance of metabolic waste from the retina.