How can I get Face ID work in a dark room?

I Face ID doesn’t require light to function, so your issue might be that at night you could be holding the device slightly differently to normal, it might be closer or not tilted the same as you would during daylight hours.

Try moving the device further away and if you are lying down sit up, also if you normally wear glasses during the day but at night you don’t, you might have to let Face ID learn that change.

I‘ve found it’s common for people to hold the device too close during unlocking (to get ready to read the screen may be), just extend your arm a little and see if things improve.

I believe a second “Face” can now be added with the latest iOS12, so try adding that at night, your device then has two to choose from which could improve your success.

Hope some of the above helps.

No problem, glasses on there own shouldn't be an issue, and I'm sure during the day you haven't really noticed, Face ID just seems to work. But at night I've found other factors come in to play, if lying down the device can end up being closer to the face when in fact Face ID works best at arms length. Also your face can get distorted depending on how you are lying (or as anypats points out, on what) which again can throw it out a bit.

Anyhow, glad you sorted it out.

I agree with all that SiHancox has said and will add that when your face is in a pillow FaceID may not be able to capture all of your face and doesn't unlock. The same can happen when lying on your side. Test it in a dark room when you aren't in bed and you'll see that the darkness doesn't affect FaceID.