I've done it often!
It's generally rattly and noisy, sometime you get a squealy carriage! ...not fun, but go with the attitude of no sleep at all and then any you get will be a bonus! Sometimes the next door lot can be noisy crashing against the adjoining door!
If you have a bargain berth you are likely to have a cabin near the loos...not so much of a disadvantage as you would think, I have certainly never heard a flush or door bang, but maybe they all had bladders of steel in our carriage?
Do keep your shoes handy for midnight/early morning pees.
Really keep your luggage to a minimum, the cabins are probably only 3ft 6 wide if that, (including the bunk) and the window wall has a sink on it, plus the ladder for the top bunk...there is no floor space for big suitcases!!
We have never bothered with visiting the dining/bar/buffet car, usually travelling with children so loath to leave them, but the guard will come and lock your cabin for you if need be, I have sent dh off for hot chocolate though, in common with the wake up coffee and tea, be warned the standards are low!
If you can just go straight to your hotel with your luggage, even if you cannot book in, it will save you traipsing your luggage with you all day, and then you can settle to the task of a fabulous breakfast! :D