The mouth spends roughly a third of each day in conditions that differ substantially from waking ones. Saliva is the variable that changes, and almost everything protective depends on it.
Saliva does several jobs at once
It buffers acid toward neutral, physically clears residue, and carries calcium and phosphate that redeposit into enamel after acid exposure.
It also contains antimicrobial proteins and forms the protein film that coats the tooth surface and mediates what attaches to it.
Because these functions are bundled together, reduced flow removes several defences simultaneously rather than weakening one. That is why dryness affects decay risk out of proportion to how minor the symptom feels.
Flow drops sharply during sleep
Salivary output is largely driven by stimulation from chewing and taste, both of which are absent overnight.
Unstimulated flow during sleep falls to a small fraction of daytime rates, and clearance of anything left in the mouth slows correspondingly.
Residue present at bedtime therefore stays in contact with the teeth for hours in an environment with little buffering capacity. The same amount of sugar behaves quite differently at midday and at midnight.
Mouth breathing compounds the effect
Air passing over the teeth evaporates what little moisture is present, and the drying is greatest on the upper front teeth where the lips part.
Nasal obstruction, allergies and some sleep-disordered breathing patterns all promote mouth breathing and therefore this drying pattern.
Persistent morning dryness, a sore throat on waking, or a partner reporting snoring are worth raising with a clinician, since the underlying cause is often treatable.
Medication is a frequent contributor
A considerable number of common medications reduce salivary flow as a side effect, including several used for blood pressure, allergy, mood and bladder control.
Taking more than one such medication compounds the effect, which is why dryness becomes more common with age and with the number of prescriptions.
Medication should never be altered without the prescribing clinician, but reporting dryness allows the dental and medical management to be coordinated.
Why the last cleaning of the day matters most
Cleaning before sleep determines what is present on the teeth during the period when the mouth is least able to defend itself.
Anything consumed afterward, including drinks that seem harmless, sits undiluted through those hours rather than being cleared within minutes.
Persistent dryness that affects speaking, swallowing or sleep should be assessed, since management depends on the cause and the associated decay risk needs monitoring.