I've travelled on sleepers several times and find the motion of the train is mostly quite gentle. I think it would be, for example, much gentler than an auto.

Many years ago, I was travelling in Italy in the middle of August. I did not know it then, but it is when the Italian military reserves are called up every year. Many soldiers came unto our train and told us we had to leave our seats (even, of course, with tickets) so they could sit. I stood in the passageway near an older woman. After it got dark, she laid her travel bag on the floor and laid down using the bag as a pillow. Since it was a night train and there were no seats, I tried it as well. It turned out to be one of the best sleeps I ever had on a train!