The structure of sleep and how it may be altered by visual impairments

Individuals with visual impairments often experience poor sleep health, which may impact brain physiology and function, as evidenced by altered brain activity during sleep. The sleeping brain can be categorized into stages: three non-rapid eye movement (NREM) stages and one rapid eye movement (REM) stage, with each stage defined by its structure, that is, the duration and frequency of specific brain oscillations. Research investigating alterations in sleep structure among visually impaired individuals has yielded mixed results: some studies indicate reduced or absent deep sleep (N3), others report longer REM latency (the time until the first REM epoch), while some suggest that circadian dysfunction may play a more significant role than visual impairment itself. Sleep is regulated by two processes: the homeostatic sleep drive, which accumulates during wakefulness and is relieved during sleep, and the circadian process, which describes the 24-hour sleep-wake cycle. The circadian process is particularly vulnerable to disruption by visual impairments, as damage to the retina can alter photic entrainment, the process by which light signals from the retina align the circadian sleep-wake cycle with the solar day. Visually impaired individuals often experience a drifting sleep-wake cycle that misaligns with the light-dark cycle, and during periods of misalignment, sleep quality may be particularly poor, especially REM sleep, which is largely under circadian control. Some causes of visual impairment, such as glaucoma, may be more susceptible to circadian dysfunction than others, as glaucoma affects cells in the retinal layer necessary for photic entrainment, which in turn may increase the risk of changes to sleep structure. Given that abnormal sleep structure is associated with long-term health consequences, including increased risks of depression, anxiety, and cognitive decline, it may contribute to the high prevalence of these issues found among the visually impaired population. Further research is needed to clarify the roles of the causes of visual impairments, circadian misalignment, and the impact on sleep structure. A better understanding of these relationships could help develop targeted interventions to improve sleep and enhance health outcomes for visually impaired individuals.