The scalp
The scalp is one of the most sensitive parts of your body when it comes to sunlight and heat. Not only because you could get a severe sunburn, but also because you can get a heat stroke.
The symptoms of a heat stroke are difficult to manage, so why risk suffering from any of it when you can just be careful? As the Skin Cancer Foundation says, wearing a hat is the best possible way to protect your head from the sunlight. If you want to add extra safety, you can spray some SPF on the top of your head or scalp to make sure you are not at risk.