Originally posted by: optimist
Shravu, it may be because it wants Prachi to suffer the after effects of its nocturnal activities. If it is permanently residing in Prachi, she will not realise the problems it has caused for her during the nights, would she? So the ghost wants to do all the evil deeds at night and make Prachi take the blame for them during the day. It will also all the ghost to take a nap during the days so that it is refreshed and extra-active during the nights.