Oh dear, I started this thread as something lighthearted, and it seems to have become very serious! I've loved all the comments from people who have or wish they had travelled on a sleeper, and to the latter I'd say, give it a go! The man in seat 61 ( seat61.com ) is the fount of all knowledge about trains-despite being a fairly seasoned traveller, I always look to him before I book any tickets.
So, after reading all your comments, I think the silk pyjamas are out and a vest top and loose shorts or leggings are the way to go.
By the way, years and ago I used to buy disposable knickers from Boots for travelling-must see if they still do them. The only problem, I seem to recall, is that they disintegrated fairly quickly in the heat. I remember hitchhiking in the south of France and arriving at the Youth Hostel wearing little more under my skirt than the elastic around the waist and legs.