Sorry, tonight I can only go as far as Wikipedia

If an upiór harassed a human at night, the remedy was to stop the upiór returning to its grave - at dawn it would disappear or change into black tar

There are two citations for that sentence but digging out the sources is too much for me tonight. Maybe there is the reasoning behind changing into tar and reference to some older sources

I think there might have been also some mention of power of the sun in some Mickiewicz work. But apparently not in “Upiór”. There it complains to Venus

But true, that is not exactly the express-sunburn OP wrote about