Sleep is critical and is necessary for optimal cognitive functioning, however sleep quality clearly changes with aging and is among the most common clinical problems in the elderly with around 40% of them complaining about sleep-related issues 1 . Dysregulated sleep may include difficulty falling asleep, but also hypersomnia, excessive daytime sleep, sleep fragmentation (more night time awakenings), sleep apnea and others 2 .
With worldwide population aging, there is an increased prevalence of dementia, defined as a group of symptoms associated with a decline in memory or other thinking skills severe enough to reduce a person’s ability to perform everyday activities. There are several types of dementia including mild cognitive impairment (MCI), Alzheimer’s disease (AD), frontotemporal dementia (FTD), dementia with Lewy bodies (DLB), Parkinson’s disease dementia (PDD), and vascular dementia (VaD). Alzheimer’s disease accounts for 60-80% of cases 1 .
Accumulating evidence suggested a close relationship between sleep disorders and dementia. In fact, sleep duration shorter than 6-7 hours/day or longer than 8 hours/day resulted in lower cognitive scores and increased dementia risk 3 4 . Furthermore, additional factors that lead to cognitive decline are poor sleep quality and poor sleep efficiency 5 6 7 .
The exact mechanisms underlying are still far from being clearly investigated. A recent study indictated that chronic sleep deprivation exacerbated the memory impairment, senile plaques depositions and phosphorylated-tau (p-tau) levels in AD transgenic (Tg) mouse model 8 . Other studies showed that aging was associated with regional brain atrophy in midline frontal lobe regions that is associated with cognitive decline.
As for the neurocognitive functions of sleep, recent research showed that restricted sleep or sleep deprivation in human was found to attenuate sleep benefits on declarative memory consolidation and increase the formation of false memory 9 .
Considering the closed relationship between sleep disorders and dementia, especially the risky and causal roles of sleep disorders in the pathogenesis of dementia, sleep-targeted treatments might be novel therapeutic strategies for dementia
Non-pharmacological strategies, such as light therapy and deep brain stimulation (DBS), might have potential application for dementia. Except these physical-based therapies, exercise regularly and daytime sleep restriction is also recommended. Intake of stimulants such as caffeine or tea and daytime in bed should be limited. Nighttime sleep should be regularly scheduled. Noise and light exposure should better be reduced 10 .
The most commonly used pharmacological interventions for sleep disorders in dementia include melatonin receptor agonists, hypnotics, antidepressant and antipsychotics 10 . However, the use of hypnotics such as benzodiazepine ≥ 60 days per year was associated with poorer cognitive function 3 . A follow-up study of 96 MCI patients found that melatonin significantly improved cognitive and emotional performance, concomitantly with improvement in the quality of sleep and wakefulness 11 . Another trial showed that add on of prolonged release melatonin to standard therapy improved cognitive functioning and daily performance in AD patients, particularly in those with insomnia comorbidity 12 .