If you are not dreaming and he is not sleepwalking, if your roommate secretly cuts your hand with a blade, it will be serious. What is your roommate's relationship with you, the roommate in the school dormitory or the roommate after work?
If you are a roommate in a university dormitory, you can talk to him first and ask him why he cut off your hand, whether he has a problem with you, or what you did to offend him. He who unties the bell must tie it. There is no enmity that cannot be untied. At school, we must try our best to keep a good relationship with our classmates. If there is a second Ma Jiajue, we are not worth losing our lives.
Another situation is that your roommate may be joking with you. Although this situation is rare, it is not impossible. I think this kind of thing is most likely to happen in the boys' dormitory if it is a joke. Boys sometimes like to play some reckless pranks. They have seen many online videos before, especially foreigners like to spoof roommates. If you hate this kind of joke, you must point it out on the spot, or your roommate will cut your hand when you sleep today and your wrist tomorrow. If you are embarrassed to point it out, you will suffer in the future.
If you are a shared roommate after taking part in the work, you know it through renting a house, and you are not very familiar with it. Under such circumstances, I advise you to change your residence as soon as possible. There is no need to say anything more. It's hard to draw a dragon and a tiger. Just stay away from him.
In any case, it is horrible to have such a roommate, so it must be handled properly to avoid future troubles.