Roles of aging in sleep.

With aging, various factors deteriorate the normal sleep process that is essential for the restoration of functional and physical performance. Due to aging-related diseases, life changes, or aging itself, disturbances in normal sleep cycles can profoundly affect healthy aging. To understand the interconnections between aging and the factors influencing sleep, with emerging evidence accumulated in recent years, this study elaborates on the roles of aging in sleep from four perspectives: cortical thinning, white matter degeneration, neurotransmitter dysregulation, and circadian disorganization. In brief, with aging, cortical thinning can be induced by the deposition of neurotoxic substances, and white matter degeneration can be induced by vascular abnormalities. These alterations emerging in the brain jointly disrupt sleep spindles and slow waves, leading to sleep disturbances. Age-related dysregulation in neurotransmitters (including galanin, orexin, serotonin, and adenosine) directly impairs the sleep modulation system. Disorganization in the circadian system consisting of suprachiasmatic nucleus dysfunction, reduced light transmission, and local circadian clock disruption collectively interrupts circadian rhythms, also causing sleep disturbances in the older. Of note is the bidirectional relationship between aging and sleep, which required us to examine this issue from different perspectives.