Here's a list of things to do to manage stress:
- Decrease or Discontinue Caffeine
- Regular Exercise
- Time-outs and Leisure
- Realistic Expectations
- Belief Systems
- Ventilation/Support System
Notice how many of these the hard-working students are NOT doing?
High stress is associated with mental illness. Here and here are two studies specifically about students (there are many more making the connection regardless of occupation), though the first of them mentions that Norwegian med school students overall don't have more mental health issues than the general population.
In conclusion: Not relaxing can lead to stress-related issues, but you don't need to specifically study quantum electrodynamics to become depressed - studying hard and worrying about passing that exam (and possibly being a foreign student in a strange culture, see here) suffices.