Introduced in 1966, the olive green (OG-106) lightweight sleeping shirt was made from nylon and was lighter, quicker to dry and more durable than the wool sweater it replaced. It also provided some resistance to insects and mosquitoes. The new shirt was primarily issued to soldiers operating in the highlands of I and II corps, where the wind-chill was significant during the dry season.1 Pilots in Vietnam continued to prefer the original wool jungle sweater due to risk of the nylon melting in a cockpit fire.
1. U.S. Army Uniforms of the Vietnam War by Shelby Stanton (Stackpole Books 1992)