How come you start feeling uncomfortable after you hold your breath for about a minute?

CO2 Narcosis: Holding your breath causes increased chest pressure which hampers return of blood to heart. Low blood in means low blood out, such as to the brain. As kids we used to play a game: we hyperventilated, held our breath, then passed out. Good thing our parents didn't know about that. Also holding the breath causes buildup of carbon dioxide and the body becomes acidic > lots of chemical changes.