Whether a person can pass gas during sleep without conscious awareness is a common curiosity that blends human physiology with the mechanics of the sleep cycle. The short answer is yes, this involuntary release of flatus, often referred to as a “sleep fart,” is a normal physiological phenomenon. Understanding why this happens requires looking at how gas is produced, the nature of muscle control, and the brain’s filtering of sensory information while resting.
How Digestive Gas is Created
The gas that is eventually expelled from the body, known as flatus, originates from two distinct sources within the digestive system. A significant portion is exogenous gas, meaning it is air that has been swallowed, a process medically termed aerophagia. This often occurs when eating or drinking rapidly, chewing gum, or through nervous habits, and is largely composed of odorless gases like nitrogen and oxygen.
The second, and often more noticeable, source is endogenous gas, which is created internally as a byproduct of digestion in the large intestine. Certain carbohydrates, starches, and fibers are not fully digested or absorbed in the small intestine due to a lack of specific enzymes. These undigested components then travel to the colon, where the naturally present bacteria ferment them.
This fermentation process releases gases such as hydrogen, carbon dioxide, and in some individuals, methane. Although most of the volume of flatus is odorless, the bacteria also produce sulfur-containing compounds, like hydrogen sulfide, which are responsible for the unpleasant odor associated with passing gas.
The Loss of Sphincter Control During Sleep
The mechanism that allows gas to be released involuntarily during sleep is directly related to the relaxation of the anal sphincters. The anus is controlled by two main muscles: the internal anal sphincter (IAS) and the external anal sphincter (EAS). The IAS is an involuntary smooth muscle that maintains a constant, high resting pressure.
The EAS, conversely, is a striated muscle under voluntary control, meaning a person can consciously contract it to prevent the passage of gas or stool. The ability to hold gas when awake relies on the coordinated, voluntary contraction of the EAS and the puborectalis muscle. During sleep, the body’s muscle tone naturally decreases, and the central nervous system reduces its control over voluntary muscles.
Studies using manometry to measure anal canal pressure during sleep have shown a progressive decrease in resting pressure as sleep deepens. This reduction in muscle tone, particularly the relaxation of the voluntary EAS, allows built-up rectal pressure from gas to overcome the weakened sphincter barrier. The lowest resting pressures often occur during Rapid Eye Movement (REM) sleep, which increases the likelihood of an involuntary release of flatus.
Why We Don’t Wake Up
The reason a person remains unaware of passing gas during the night is due to a neurological process called sensory gating. Sensory gating is the brain’s mechanism for filtering out redundant or irrelevant stimuli to prevent an overload of information. This filtering process is significantly amplified during sleep, ensuring that minor internal or external disturbances do not interrupt rest.
The brain effectively raises the threshold for sensory input, including internal stimuli such as the slight pressure change or sensation of a small volume of gas being expelled. Only a stimulus that is particularly intense, such as a loud noise or a significant sudden increase in pressure, would be substantial enough to cross this elevated sensory threshold and trigger an arousal or waking response.
The brain is continuously analyzing the preserved sensory input, but it is primarily looking for stimuli that indicate danger or are personally relevant. For most people, the minor feeling and sound associated with passing gas do not meet the criteria to penetrate the deep sensory filtering of the sleeping brain. The process of sensory gating protects the continuity of sleep by essentially muting the body’s minor functions, making the expulsion of gas a completely unconscious event.