I just successfully completed my first year for a Ph.D program in a top 10 Ivy league school and I must say it was the hardest year of my life. I would work 12 hours a day and on average spend 8 hours on a single problem in my problem sets. Luckily I am done with all of my classes and quals and just need to work on research. From your experience do you think the the worst is behind me or do you think on a yearly basis doing research and working on your dissertation is even more stressful then taking required classes that you had to pass or else you be out of the program? My dissertation is going to be in theoretical particle cosmology and I have a clear idea what direction I want to take it.