Hell, I often travel wearing a solid colored (no stupid logos) half sleeved tshirt and track pants - not gym wear, just comfortable enough to wear to sleep on a flight that leaves MAA at 1 AM and lands in FRA quite early in the morning. Smart Casuals are wasted on a flight like that. I do change into business casuals after a shower in the lounge, for the day flight stateside or wherever I am connecting to on LH. Comfortable suede loafers / slipons.

Inappropriate be damned, I am not wearing smart casuals on a past midnight flight when I'm just going to sleep in them. I like my smart casual and formal wear fresh ironed, and I dont pack along all that much when I am traveling, so.. no sense wasting them to sleep on a midnight flight.

Come to think of it, the americans, germans and anybody else traveling in F, J and Y on that flight tend to be casually dressed in old and faded clothing.