Possible answers to a questions on the lines of "Do you mind taking the Upper berth? Actually we've kids accompanying us!":
.
1: "Ah I would've surely sir but actually I got this leg injury 10 years ago and I've a metal rod placed in my left leg, Doctors have strictly asked me not to stress it so I'd be unable to"
2: "The...
more... vents! The AC vents are too close on the UB's sir! I've this allergy to even slightest of dust and I cant stand too cold a temperature! I'm sorry."
3. (mumble this in their ears) "The bowels are not doing too good sir! Visits to the washrooms are expected, please don't bother me with a climb-up-and-down exercise *wink* *wink*"
Other explanation may include use of terms like "knee-fracture", "pregnancy (conditions apply)" and "Acrophobia". Suit your case. All the best. (y)