BACKGROUND: Previous subjective data suggests that shorter total sleep time is associated with greater odds of inadequate hydration, but it is unclear how variability in fluid intake alters objective sleep metrics. Thus, the purpose of this study was to examine the relationships between 3-day variability in individual daily fluid intake, hydration status, and objective sleep metrics. METHODS: Twenty-seven healthy males (mean ± SD; age, 23 ± 4y; height, 176 ± 6cm; weight, 78.7 ± 12.9 kg; body fat, 16.6 ±8.7%) collected 24hr urine samples to assess mean hydration status (Urinary Osmolality (UOSMO)) for four consecutive days. Participants self-reported habitual fluid intake using a validated log (Liq.In.7) and sleep was captured over the same period using actigraphy. Separate random-intercept linear mixed fixed-effects models assessed the effects of within-subject variability and between-subject differences in daily fluid intake (L), UOSMO, on objective sleep metrics (total sleep time, efficiency, latency, average awakening length). RESULTS: Every additional 1L increase in daily fluid intake above grand mean (2.9 ± 1.7L), was associated with a 3-min increase in sleep latency (β = 3.35, [0.7, 4.7], p = 0.009]). On days when a person’s UOSMO was higher than their typical UOSMO, they experienced a longer duration of awakening bouts (β= 0.005, [0.003, 0.009], p = 0.031]. There were no associations between fluid intake with total sleep time (β= -0.03, [-0.580, 0.037], p = 0.679), sleep efficiency (β= -0.01, [-0.053, 0.023], p = 0.454), wake after sleep onset (β= 0.03, [-0.166, 0.245], p = 0.714) or hydration status with total sleep time (β= -0.23, [-0.517, 0.061], p = 0.130), sleep efficiency (β= -0.01, [-0.040, 0.010], p = 0.242), wake after sleep onset (β= 0.05, [-0.080, 0.197], p = 0.466). CONCLUSIONS: Greater daily fluid intake affects onset of sleep, but not total sleep time, sleep efficiency, or wake after sleep onset. Inadequate hydration compared to one’s usual hydration status may negatively affect sleep outcomes. Day-to-day variability in fluid intake should be considered in longitudinal research to help contextualize biological and behavioral modifiers of hydration status and its subsequent effects on sleep quality and quantity.

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