I'm currently an intern. You should plan to work hard during internship no matter what specialty you go into.
Another thing you have to prepare for about internship is that it's not always as if you can punch in and punch out right at your scheduled time like you can in many other jobs.
If there is a code a few minutes before you are scheduled to be done with call, it's not like you can just walk away saying you're off the clock. In clinic you may have to stay several hours after you're done seeing patients to document the visits.
Or the residency program may have to change the call schedule for one reason or another if your other residents get sick/pregnant/etc., as I just experienced. I am an outpatient IM month, which is relatively relaxed (9 to 5 most days), but thanks to a last minute schedule change I wound up having to take a lot of call this past week.
I was on call at the hospital Thursday night through Friday morning (14 hours), Sunday morning to Monday morning (24 hours straight), and then Wednesday night through Thursday morning (14 hours again). After taking call, you are often expected to stick around another six hours (either in educational conference or on your regular rotation) and that is actually the hardest part. I fell asleep in the educational conference I had today at 8 after being on call last night.
Even though I had Saturday off, I was still kind of tired from Thursday's call and wanted to rest up for Sunday's call so there is no way I could have had much of a life this past weekend.
Being a doctor is hard work, and it's a different sort of hard work than most other jobs (not many other jobs where you're kept sleep deprived and you have to make decisions that could potentially kill someone). You are going to work hard and be under a lot of pressure for a long time during medical school and residency. You have to try to get as much exposure to medicine beforehand to be sure it's really worth it to you.