For me, it was a little bit of both. I started out thinking that I was smarter than the average bear and that I was going to cure cancer, find infinite energy, and have the ability to fly all before the age of 30 and then start a career as a professional surfer. Along the way, I had to make a few minor modifications.
For the first year and half to two years in grad school I was probably working 70-80 h a week. In retrospect, it was probably because there were some subtle indicators via word of mouth that the professor would kick you out of the research group if you were lazy rather than a passionate interest. During my second year, a Chinese guy joined the group and he put in about 30 h a week at work and goofed off the rest of the time. I thought he was going to get booted, however, after watching him work, he was probably the most efficient person I had ever seen. I watched and learned a bit and then started cutting my hours back to about 40 per week. After graduating, I became very disillusioned again upon learning that most "good" research jobs in industry or opportunities as professors were reserved for the "elite" (i.e. MIT or Cal tech grads). I probably spent a year just trying to get a job interview. By the time I finally got an interview, I was so PO'd that I just put down a high amount of money (90 K US) knowing that the employer wouldn't match it as I started planning another career. Oddly, not only did the employer did match it, but I've been getting ~15 % raises for the last 6 years. With the money I've saved from the income, along with a few lucky investments in Taser and Valero, I could probably retire today. Today, it's all about money and science is just a blip along the way. I like to think that the analytical thought rigor that I have gained along the way came from those 80 h weeks, however, it might be innate and those hours were just wasted time. I try not to think about it much now.