Radiant heat from solar energy is actually radiation which is capable of penetrating deep into bone tissue and exciting chemical reactions. It's far different from just plain old warmth, it's actually radiation which creates biochemical reactions in the bony armor plating.
Rather like how the exact same temperatures feel much different when sunlight is involved. You can be so hot you are sweating like crazy in direct sunlight when the exact same temperature indoors keeps you very cool. It's all about the radiation.