Perhaps some sort of heatstroke mechanic. As with real life, if you are out in the sun nonstop with no shade, you will eventually die. I would suggest another bar to keep watch on, like food and water, but going down much slower. Spending time in the shade would reduce the chance of getting heatstroke - Maybe you could be able to build canvas/tarp things out of thatch as a shelter, because it would be nice to have a roof that isn't made out of planks.