Open-Mouth Sleep and the Case of the Missing Water

Comparing 60 people with obstructive sleep apnoea to 15 healthy controls, this study used overnight infrared monitoring alongside airflow sensors to track exactly how much of the night each person spent with their mouth open, then measured blood samples before and after sleep to estimate fluid loss. The OSA group spent almost the entire night with an open mouth, compared with roughly a quarter of the night for healthy controls, and showed correspondingly greater overnight increases in blood concentration, a marker of dehydration. The more time spent in complete mouth breathing, the greater the estimated fluid loss, and this pattern held even after accounting for the severity of the apnoea itself.

It's a modest-sized study, and it can't say whether the water loss itself worsens next-day symptoms such as headache or fatigue, only that it accompanies open-mouth sleep. It's also drawn from an OSA population rather than healthy mouth-breathers, so the scale of fluid loss may not transfer directly. Still, it offers a plausible, measurable link between an open mouth overnight and simply waking up more dehydrated.