Effects of Fluid Intake on Sleep Duration and Quality Among Healthy Adults.

Purpose: Inadequate fluid intake disrupts body homeostasis. The aim of this study is to examine the relationship between fluid intake and sleep quality/duration among healthy adults.

Participants and methods: This crossover study included 15 healthy adults aged 18-40 years. Participants completed an initial study visit followed by 2 four-day monitoring periods separated by a one-week washout period. The first monitoring period (P1) was used to establish a baseline hydration level. In the second monitoring period (P2), participants reduced fluid intake to induce mild dehydration. Sleep was continuously monitored during P1 and P2 using a wearable sleep sensor (Oura Ring^®), while hydration status was assessed via 24-hour urine samples and urine-specific gravity measurements. Data analysis included paired sample t-tests, correlation, and mixed model analysis to examine relationships between variables.

Results: There were no significant differences in sleep variables between hydration protocols. In a subset of successfully dehydrated participants (n = 8), sleep length, REM length, and sleep efficiency all associated with quantity of water/fluid intake (p < 0.05), with a significantly strong correlation found for REM sleep length (r = 0.800, p < 0.05, R²= 0.64). In the linear mixed effects model, a significant interaction was observed between adequate hydration and water intake on REM sleep (B = 37.41, SE = 9.89, unadjusted p = 0.006, adjusted p = 0.028). Similarly, a significant effect of water intake on REM sleep in the dehydration condition was observed, however, after applying the Bonferroni correction the effect was no longer significant (B = 30.04, SE = 12.45, unadjusted p = 0.043, adjusted p = 0.22).

Conclusion: Mild dehydration does not appear to affect sleep measures in healthy adults. However, fluid intake correlates positively with REM sleep length, sleep duration, and sleep efficiency, suggesting that the quantity of water consumed may influence sleep quality. These findings highlight the potential benefits of adequate fluid intake for optimizing REM sleep, which is vital for cognitive and overall brain health.