I agree with Marc. In fact, when a snake has a significant lump in their belly, they actually get MORE exposure from above than they do from below. Because the stomach has expanded, there is just as much in contact with their back as their belly, so if snakes do bask when digesting (I was under the impression that most snakes, or most colubrids anyway, hide after eating, because they become more sluggish and vlunerable), it's probably because they get both the belly heat and the overhead heat. Can't get more efficient than that.
- Ken LePage