@Criminal_Bizzybut under trance PHB says “After resting in this way, you gain the same benefit that a human does from 8 hours of sleep.” Sleep isn't the same thing as a long rest. Elves don't need sleep, but they do need downtime to regain their strength.

— Jeremy Crawford (@JeremyECrawford) January 7, 2015

@Criminal_BizzyPHB says that a long rest is “at least 8 hours long, during which a character sleeps or performs light activity:” = Trance The intent is that Trance replaces sleep, not the light activity that can fill part of a long rest.

— Jeremy Crawford (@JeremyECrawford) January 7, 2015