More skin coverage doesn’t necessarily equate to feeling hotter during a race. Fabric around the shoulders and arms can block the sun and help keep the skin damp between aid stations. Research hasn’t measured a change to core temp when wearing arm sleeves in hot weather, but the psychological effect has been clearly observed. On the other hand, pro triathletes Matt Reed and Luke McKenzie both felt comfortable racing a full-sleeve kit in hot Ironman races. If you’re worried that covering your shoulders will affect your race, it’s best to stick to sleeveless.