Why the Mouth Opens Wider in the Deepest Stages of Sleep

Using sensors that tracked jaw position alongside a standard overnight sleep study, researchers followed 42 patients referred to assess snoring and suspected obstructive sleep apnoea. Mouth opening turned out to be far from random: it was smallest during light sleep, greater during deeper stage-two sleep, and widest of all during REM sleep, when muscle tone throughout the body drops to its lowest point. The pattern also differed depending on whether a person's breathing problems were worse in REM or in non-REM sleep, hinting that mouth opening may itself play a role in why some people's apnoea clusters in particular sleep stages.

Because everyone in the study had been referred for suspected sleep apnoea, the findings describe this population specifically rather than healthy sleepers in general, though the underlying mechanism, jaw muscles relaxing further as sleep deepens, is a normal physiological process rather than a disorder. The useful takeaway is that a wider-open mouth in the small hours may simply reflect being in deeper, more restorative sleep, not a sign that something has gone wrong.