Evaluating the Effects of tDCS on Sleep and Cognition in Healthy Older Adults: A Randomised Controlled Trial Using a Nap Design

Sleep is vital for maintaining physical and cognitive health across the lifespan. However, normal age-related changes to sleep can impair memory, learning, and waste clearance in the brain, contributing to the risk of cognitive decline and dementia.

Transcranial direct current stimulation (tDCS), a non-invasive technique that modulates brain activity, shows promise for enhancing both sleep and cognition. However, existing studies have significant methodological limitations. Notably, very few have investigated 'offline' tDCS -where stimulation is applied before sleep- to assess objective sleep changes using polysomnography (PSG). Instead, most research has relied on subjective measures, consistently finding that offline tDCS enhances perceived sleep quality, especially with repeated stimulation sessions. Importantly, offline tDCS may be more feasible for use outside laboratory settings compared to 'online' approaches, where stimulation is applied during sleep, making it a more scalable intervention.

Furthermore, delivering tDCS before a daytime nap, rather than nocturnal sleep, offers additional advantages by reducing participant burden, particularly in older adults. Studies applying tDCS during a nap in older adults have shown improvements in objective sleep microstructure and declarative memory. By contrast, studies applying tDCS during nocturnal sleep have reported inconsistent results. Despite its potential benefits, no studies have explored the effects of an offline, nap-based tDCS approach on objective sleep outcomes.

Additionally, few studies have examined whether repeated sessions of offline tDCS might amplify its effects on objective PSG-measured sleep. While repeated sessions have been shown to positively influence subjective sleep outcomes, it remains unclear whether the same benefits apply to objective sleep metrics. This study will address this gap by investigating whether two sessions of offline tDCS, compared to one, are necessary to improve objective sleep outcomes.

In summary, this study aims to determine whether offline tDCS applied prior to a nap can enhance objective sleep during a daytime nap and improve post-nap cognitive performance in older adults. If effective, this low-burden, non-invasive intervention could serve as a practical tool for promoting healthy aging and reducing dementia risk.