Ruyan,

True, it is very necessary to have enough sleep otherwise, your performance will be affected.

Remember that you have been assigned a mentor, and it is an opportunity to ask for tips and advice on how to cope with or tackle some challenges. These mentors have gone through the same transition process.
So, I hope you are taking advantage of the extra help you could get by asking your mentor for any tips. Even for next year's courses which you could start preparing for during the Christmas holiday. 

Good luck with your exams.