Less saliva is produced during sleep than during wakefulness, possibly partly related to a reduction in oromotor activity, but the volume is similar in NREM and REM sleep. The sensation of a dry mouth is common during sleep, particularly in the presence of snoring and sleep apnoeas.
The swallowing frequency is similar to that during wakefulness in stage 1 NREM sleep, but in the deeper stages is less frequent. Swallowing may occur without causing any electroencephalogram features of arousal.
Gastro-oesophageal reflux commonly occurs during sleep, and clearance of acid from the oesophagus is slowed, probably because of a reduction in swallowing and oesophageal peristaltic activity. There are rhythmic gastric contractions approximately every 20s in NREM sleep. The interval between peristaltic contractions in the small intestine and colon is increased in NREM sleep compared to wakefulness and there is an increase in colonic activity after waking .
Changes in the stability of the anal sphincter have been found during sleep.