what caused the sun weakness and deathsleep in vampires?

I've been listening to the audio books and reading up various material regarding Amel, Akasha and the lore of the vampires. One thing I can't find a source point on is, what caused the sun weakness and deathsleep when Amel blended his spirit with Akasha's soul?

As I understand it from the forums and wikis, replimoids have no weakness to the sun, nor did Amel as a spirit (just the blood thing). So why would Akasha go into a deathsleep or burn (did she burn at first or did the burning only come to new vampires as the core was diluted among fledglings?) from the moment she became the first vampire?