It may look stupid and I dont even know if you can buy them anymore, however I have definitely proven this theory correct. Last year while sailing in "bloody cold" Bunbury I would go out in a sealed-seam full wetsuit. Still feeling the cold I pulled out the old wetsuit hood and before I knew it, I would be breaking into a sweat out in the rain and hail. When I go to hot, I just pulled the hood down and around my neck. As they say...most of the heat you lose is through your noggin! What ever it takes to give you more time on the water.
Happy sailing