You use 'while' when you are doing something first (1st event) and something else happens during that time( 2nd event). While I was sleeping , my girlfriend check my wallet / While I do not think its important (1st) , he just kept quiet (2nd).
Whereas > A word that is used as a conjunction to means different or contrary to what was said earlier . I like girls whereas he likes guys/ People here are mostly drunk whereas those in that place are still sober