+100
I'm in IM, and I can tell you that residency is massively more stressful. If I look back at med school or even college, I do remember a lot of stress associated with grades - 'will I pull it off to get to the next level' etc - but none of this compared to the deeper, more insidious stress you will feel as a resident. You will feel the full brunt of the responsibility for caring for someone's life. Postcall, you will be running through decisions you made over and over again wondering if it all was right etc. You will fully appreciate the sheer volume of stuff you have to learn. As I frequently tell the med students - I generally feel like I'm learning (and/or relearning) about the same amount of book material from med school, just crammed into whatever time remains after you work 80 hrs/week. As stated above, it's one thing to learn basic disease concepts but when you move to fully understanding the depths of current literature, you add a whole new dimension to your required learning. Furthermore, unlike med school there's usually no one 'bible' textbook of your specialty that everyone just memorizes etc - your learning becomes this hit-and-run pastiche of stuff you picked up while googling, random facts gleaned from a journal article or grand rounds session, things an attending taught during a 'chalk talk', street smarts that you pick up on overnight call, facts your senior resident chastised you for when you were an intern, etc. You'll realize how far you've come the first time it dawns on you that you're on a totally different plane vs the medical students and can effortlessly pimp the **** out of them (I don't do this, btw).
(And, to some extent, you SHOULD feel the stress of this responsibility as a resident and you should not be shielded from it by overly involved attendings or senior residents. Otherwise, you're going to be in for a very rude awakening when you head off as a fresh attending.)
You'll feel this stress not only in the ICU but in the clinic. That random pt that came to you on cyclobenzaprine, tramadol, and a bazillion other medications with scripts being filled from 3 different doctors whom your co-resident mindlessly started on an SSRI in clinic today? He's gonna develop serotonin syndrome in a few weeks and seize (this actually happened, btw). You'll do an ICU rotation for a month, get (relatively) good at adjusting vents/pressors/etc, go off and do a bunch of other rotations for 8 months, then come back on an ICU rotation and realize most of those skills have faded and you have to relearn things again. You'll most likely be doing multiple consecutive blocks of call where you'll go without sleep every 3rd-5th night for months on end. Your senses will be dulled, your memory will be diluted, your ability to focus can be compromised and you'll be exhausted. You'll admit people over a 28hr call shift and then have to defend everything you did all night when you are at your absolute nadir in terms of wakefullness. You'll see totally reasonable and acceptable management decisions get trashed by a random attending who only does things some other way and will not admit that there's another legitimate way of managing pts etc (usually happens on the days attendings switch! If I had a nickel for every time one attending thought something done by some previous staff/team was 'inappropriate' or 'malpractice' etc I'd be able to quit and move to Costa Rica).
As a 1st year, you'll be trying to get your footing in everything. As a 2nd year, you'll gain a lot of knowledge and confidence but during 3rd year it'll dawn on you that you are mere months from practicing and you have to make damn sure you're comfortable with doing everything...it's overwhelming at times.
And the 'carnage factor' discussed above certainly exists too. It really sank in at our VA, where the sheer sense of patients' self-neglect seems to be constantly present - when you admit one new terminal lung cancer after another for a month with dudes that smoked 3 packs/day for decades and are still puffing away, things can get a bit...discouraging.
In short, you'll finally be elbow deep in patient care in a way that was impossible as a medical student. I love it, but as you've probably gathered, it has its ups and downs. And it's way harder than medical school. You might not have well-defined 'tests' constantly like you did back then, but your knowledge and skills will be tested more acutely in real-world conditions every single day.