My vampires usually take a night to earn level 10 on 1 skill. But I had this one once that was out in the sun for a while, a sim hour or so. And she would feel weak, and wouldn't even learn skills with the little flying bat over her head. And her needs bar would drop a lot faster. I kept her in doors, well underground, for 2 days and everything seemed back to normal. So it could be do to excessive sun.
(Sorry about bad grammar, english is not my first language.)