I studied super hard in undergrad, because I knew it was my ticket into Dentistry and that is all I've ever wanted to do with my life. I've never been the type to study constantly though, even in undergrad I never studied every single night or reviewed my notes (save for 1 or 2 classes) after lecture. I did put a ton of time into conquering the tests though, I use to study 20 hours for every midterm in the days leading up to it. I did rather well, finishing with 4.20/4.30 GPA for my undergrad career.
In dentistry, I have found that studying that amount is simply not required. I also feel like I've missed a piece of my life that I want to experience, as I have more friends that like to go out and such now. I still study before midterms and exams, and put a ton of time into projects. But I don't study every night after school (which is 8am to 5pm every day of the week), its very difficult to come home from a long day and still be able to concentrate and really assimilate material at that point. By the time my girlfriend and I eat and clean up it's usually 7pm, and that only leaves a 3-4 hours before bed. I'd rather get a good sleep at night, and be ready to be attentive and alert during class the next day. Most of my class brings their laptop and just goes on facebook or random websites while lecture is happening... I never bring mine, and always try to get something out of lecture. I still get *good* marks, I've had almost all A's in Dentistry so far (starting year 3 in August) and am at 3.77/4.30 GPA. And I've had infinitely more fun doing it, I was almost a nervous wreck after undergrad.
If you go to a clinical intensive school like me, all you really need to do is make sure you know what you are doing in the chair with a patient... review the procedures the night before until they become 2nd nature, and you will learn well from those experiences. Dentistry is not difficult, you just need to have a conscience that tells you how to respond to deviations from the ideal sequence of a procedure.
If you can study for 3 weeks for the DAT, without really studying a lot in undergrad, and still do well, then you obviously can absorb a lot of information in a short time. Just read your notes and stuff before midterms, and you will be fine.