Why Do We Fart in Our Sleep?
Nighttime flatulence is one of those biological certainties that polite society refuses to discuss. While you're asleep, your body doesn't stop producing intestinal gas — it simply stops caring about the social consequences of releasing it. The science behind nocturnal flatulence involves sphincter relaxation, gut motility patterns, and the fascinating fact that your nose is also partially off-duty during the night.
The Relaxed Sphincter
The external anal sphincter is a voluntary muscle — meaning its resting tone is maintained by conscious neural input. During sleep, particularly during the deeper stages, the voluntary control of the external sphincter diminishes significantly. The muscle's resting tone drops, and gas that has accumulated in the rectum during the day — or that continues to be produced by the gut microbiome overnight — exits with little resistance. This is the primary mechanical reason nocturnal farting occurs. It is a physiological inevitability: every healthy adult flatulates during sleep. The question is only one of frequency, volume, and whether anyone is present to observe it.
Gut Motility at Night
The gastrointestinal tract does not enter sleep mode along with the rest of the body. Intestinal motility continues throughout the night, regulated by the enteric nervous system — sometimes called the 'second brain' — which operates largely independently of the central nervous system. During sleep, the migrating motor complex (MMC) sweeps the small intestine in recurring cycles of approximately 90 minutes, propelling contents toward the colon. Colonic motility also continues, and fermentation by gut bacteria never stops. Gas produced throughout the night accumulates and is steadily released, with the overnight peak typically occurring in the early morning hours when gas production from the previous evening's dinner has reached maximum output.
REM Sleep and Gas Release
Sleep architecture affects flatulence patterns. During REM sleep, skeletal muscles throughout the body experience pronounced relaxation (atonia), which extends to the pelvic floor musculature. This makes REM sleep periods particularly associated with gas release events. Research on anorectal physiology during sleep shows that the rectoanal inhibitory reflex — where internal sphincter relaxation occurs in response to rectal distension — remains active during sleep, while the voluntary override mechanism is reduced. The result is a more permissive environment for gas passage. Partners who claim to have 'never' heard their significant other fart are, statistically speaking, simply heavy sleepers.
Smell Detection During Sleep
A curious corollary of nocturnal flatulence is that olfactory sensitivity is substantially reduced during sleep. The olfactory system does not completely shut down — certain intense odors can still rouse sleepers — but the threshold for odor detection increases significantly during NREM sleep stages. This means that mild-to-moderate nocturnal flatulence is often not detected by the person producing it or, indeed, by a lightly sleeping partner. Studies on sleep and olfaction confirm that only sufficiently concentrated or qualitatively distinct odors penetrate sleep consistently. This anatomical quirk provides a modest evolutionary mercy for co-sleeping arrangements worldwide.
Did You Know?
- The enteric nervous system contains approximately 100–500 million neurons — roughly as many as the spinal cord.
- The migrating motor complex cycles every 90 minutes during fasting and sleep, sweeping intestinal contents forward.
- Olfactory threshold (odor detection sensitivity) increases by up to 50% during NREM sleep stages.
- Gas produced overnight tends to have higher concentrations of hydrogen sulfide due to prolonged bacterial fermentation.
Fun Fact
A 1990 study by Orkin et al. published in Gastroenterology used continuous anorectal manometry during sleep to document that anal sphincter relaxation episodes — the events that allow gas to pass — occur at a rate of roughly 5 to 15 times per hour during sleep, dramatically higher than the waking rate.
Sources
- Orkin, B.A. et al. (1990). Temporal variation in anorectal function. Diseases of the Colon & Rectum, 33(4), 315–318.
- Kumar, D. et al. (1990). Prolonged anorectal manometry and external anal sphincter electromyography in ambulant human subjects. Digestive Diseases and Sciences, 35(5), 641–648.
- Carskadon, M.A. & Herz, R.S. (2004). Minimal olfactory perception during sleep: why odor alarms will not work for humans. Sleep, 27(3), 402–405.
- Sarna, S.K. (1991). Physiology and pathophysiology of colonic motor activity. Digestive Diseases and Sciences, 36(7), 998–1018.