in my 3rd year as a PhD and couldn't have made a better decision - during my last 2 years in undergrad i interned for an organization and even worked for them after graduating for a year before transitioning to the PhD. Oddly i felt the opposite affect - when working full time i was drained every day, weekends were the only refresh, while on the PhD i work more (in terms of hours), but feel more refreshed as i build my own schedule and get to focus more on what i'm interested in as opposed to balancing paperwork, meetings, and politics.