[Alterations of physiological rhythms in neurodegenerative disorders: problems and prospects of light therapy]

This review provides an outline of circadian disorders associated with neurodegenerative diseases of the brain in the elderly, including Alzheimer's (AD) and Parkinson's diseases (PD). Since light is one of the most powerful environmental cues synchronizing physiological rhythms, exposure to bright light is often used to treat sleep disorders, behavior of patients with severe dementia, gait, and other functions disordered in CNS pathologies. Photobiomodulation effects of the low-energy laser and LED radiation is also considered as a potential tool for the rehabilitation of patients with various pathologies. It yielded positive results in the treatment of both PD and AD. An external factor that can exacerbate a course of neurodegenerative diseases is light pollution. Irrational artificial lighting can cause jet lag, affect visual and cognitive functions. The review also summarizes the current understanding of fractal dynamics of physiological rhythms in healthy subjects (including brain activity) and their reduced complexity in pathological conditions. The loss of multiscale complexity of brain electrical activity has been documented for patients with AD, PD, epilepsy, and schizophrenia. It is thought to reflect fundamental changes in the regulation of the body's circadian rhythms. Recently, M.V Zueva proposed and theoretically substantiated the hypothesis that allows taking another look at the prospects for the use of light stimulation. It is expected that the use of stimuli with fractal structure can help to restore the retina and brain’s functions in neurodegenerative diseases by reactivation of neuronal plasticity and be an innovative method for the treatment of neurodegenerative disorders.