Please know that night fall is a normal phenomenon in young people like you. You seem to have a lot of negative emotions about these episodes which is troubling you and making you feel less energetic,lazy and affecting even your studies. Discuss whats on your mind with a family doctor or a sexologist
who will reassure you.Whats important is to relax and get your mind in order as your problem is nothing to be worried about or guilty about.
Hope this helps.