Sleep-wake control with age and neurodegenerative diseases

Aging causes dramatic alterations in bodily functions. Among them, sleep quality declines with age, particularly in individuals with neurodegenerative diseases. In this review, we first describe alterations in sleep-wake architecture and discuss potential mechanisms underlying sleep disorders that arise with age. We discuss evidence linking sleep disorders with neurodegenerative diseases such as Alzheimer's disease (AD), Parkinson's disease (PD), progressive supranuclear palsy (PSP), Huntington's disease (HD), and amyotrophic lateral sclerosis (ALS) syndrome. Even though the causes of AD, PD, PSP, and HD are diverse, several shared symptoms including difficulty falling asleep, fragmented sleep, and disrupted circadian rhythm collectively suggest their pathologies disrupt sleep-wake control. Hyperexcitability of implicated neurons is commonly observed prior to neurodegeneration. Upregulated neuronal excitability in the early phase of these diseases appears as a potential shared mechanism among neurodegenerative diseases. Abnormal protein accumulation and aggregation in these diseases exacerbate neuronal circuit hyperactivity by increasing neurons' intrinsic excitability or dampening inhibitory inputs to neurons controlling sleep-wake cycles. A better understanding of the mechanisms underlying sleep disorders that emerge with age may greatly benefit the development of novel preventative and therapeutic strategies for neurodegenerative diseases, and therefore improve the life quality of older adults.