PURPOSE: Inadequate sleep and underhydration have been independently associated with adverse health outcomes. However, the relation between hydration status and sleep has yet to be investigated over the course of several days in young adults. Thus, the purpose of this study was to assess the association between 24h urinary hydration markers and both perceived and objective sleep quality.
METHODS: Eighteen participants (female, n=7; age, 23±3; height, 174.6±15.3cm; body mass, 73.5±15.9kg; body fat, 19.4± 9.4%) provided a 24h urine sample on seven consecutive days for measures of urine volume (U_VOL), urine osmolality (U_OSMO), urine specific gravity (U_SG) and urine color (U_COL). Objective sleep metrics (wrist-worn actigraphy) and subjective sleep assessments (Karolinska Sleep Diary) were recorded each day. Actigraph measures included periods of wakefulness after defined sleep onset (WASO), sleep time, wake time, and sleep efficiency. The Karolinska Sleep Diary included nine questions used to assess perceived sleep quality from the previous night. Mean values were calculated for each participant for all variables on weekdays (Monday-Friday) and weekend days (Saturday/Sunday).
RESULTS: Higher weekday U_SG and darker U_COL were both associated with greater weekend time spent sleeping (U_SG, adj R² = 0.203, p = 0.024; U_COL, adj R² = 0.274, p = 0.015). Decreased weekday U_VOL was associated with increased weekend time spent sleeping (adj R² = 0.220, p = 0.028). Higher weekday U_OSMO was associated with greater weekend WASO actigraph measures (adj R² = 0.205, p = 0.045).
CONCLUSIONS: Mean 24-hour urinary hydration markers depicting a state of underhydration (elevated U_SG and reduced U_VOL) across weekdays were associated with an increased number of awakenings during the weekend nights, albeit, having a longer sleep time. Determining how day-to-day variations in hydration status and other general health behaviors influence sleep has yet to be explained.