Dunking your face in a bowl of cold water while holding your breath triggers the diving reflex, which slows the heart rate, redirects blood flow toward the brain, and contracts the spleen to release red blood cells, providing an oxygen boost of approximately 10%.

noli's read

The diving reflex described by researcher Melissa Ilardo is a robust and scientifically validated physiological phenomenon. Experimental studies led by Erika Schagatay's team (2001, 2015) confirm that holding one's breath with the face in cold water triggers a contraction of the spleen, releasing red blood cells to optimize oxygen supply. Furthermore, a clinical trial published by Espersen et al. (2002) shows that this contraction temporarily reduces the volume of the spleen, which increases the blood's oxygen-carrying capacity. However, characterizing this oxygen boost as '10%' for everyone is slightly exaggerated: in the majority of untrained individuals, the increase in circulating red blood cells is closer to 3% to 6%, with the upper limit reserved for highly adapted profiles. Finally, research shows that this mechanism is not instantaneous upon first contact with water, but generally requires several repeated breath-holding cycles for the spleen to fully contract.

the sources

No named source held this one up — that absence is part of the verdict.