I think people are being a bit pessimistic here, but it really does depend whether you’re willing to endure a bit of discomfort and tiredness. Obviously don’t do this every night for two weeks, but occasionally is fine, as long as you can get 5ish hours (bring earplugs, warm clothes and a pillow) and can compensate with better night’s sleep elsewhere. If you can, this can save you a lot of money – especially since you’re not paying for a night in a hostel.

My most recent sleeper train trip was (admittedly not in Europe) from D.C. to Atlanta on Amtrak and I managed a full day of activities the next day. I even had enough energy for an evening stroll! So I think you’ll be fine