What Do You Sleep In
 

We are flying prestige class for 15 1/2 hours from US to Seoul. Then another 6 hours to Bangkok.

Do people take their night clothes and change into them in the bathroom on such long trips. I was thinking about taking my nightgown and robe.

Airline-supplied PJs if they are any good, otherwise loose cotton/linen trousers and t-shirt. Change in toilet before take-off.

Sometimes change into PJs after the meal service or just wear comfy clothes onboard.
KE Prestige doesnt offer PJs unfortunately.

Dont recommend changing into PJs before takeoff although it is convenient.
This is because if there is a takeoff problem (very rare of course) and if u need to evac, wearing PJs would suck.

So it wouldn’t be frowned upon as I march to my seat from the bathroom in my nightgown and robe?

Maybe just casually walk? Avoid the march. ;)

KAL keeps their planes hot. I try to change into Lululemon vented t-shirts and shorts to sleep.

I usually pack comfortable clothes or a set of pajamas (depending on airline) in my carry on and change after take off. As others have mentioned, KE keeps the cabin warm. On my last flight I had basketball shorts and a tshirt and was still sweating :(

Airline PJ's if provided. If not, I usually travel with some LH F PJ bottoms which are basically OR scrubs and sleep in those and a tshirt. Change in the lav after we get to cruising altitude and change back before landing

It depends on the nightgown. If it's sheer, low cut, or very short, sleeping in it in "public" wouldn't be decent. OTOH, if it's a granny type neck to toes thing, it's likely to be hot and probably would lead to some odd looks.

Why not just comfortable pants/shorts and a tee shirt or other loose and comfortable top?

If you feel comfortable that way, then go for it. I'm not sure I'd get into my boxers and Hugh Hefner robe on an aeroplane, but that's just me.

Stay "Normal" and be your self Please!!!
I, myself try to dress easy/comfortable/normal, yet slightly formal during BC flights - and nothing special.
BUT I se all kinds of people onboard. Too bad so many full/Complete Families INCL noisy Children are flying BC these days. BC is the new "Long leg Economy Class" these days it seems IMO.
Anyway, MOST important - behave polite and good!
Drink your wine, Brandy, water or whatever - show your nice watch or whatever - BUT generally, other people do not care, believe it or not. YOU are just "a seat in the air" after all.
Relax. ENJOY the hours in the air. Soon you are down to Earth as most anyway.
You know - you meet ANY kind of people up there in the air - even other Prestige Class Passengers. Ehh, surprised?

Good flight,
Bent

I think I will wear my nice clothes for boarding and when we reach altitude go and change into a pair of light sweat pants and a light t-shirt to sleep in. I’ll change back before we land. That way my clothes stay relatively fresh and I will be more comfortable in order to get some decent sleep.

have to say this post made me laugh!!!
I'm a real man, I don't wear pjs and I certainly don't own any!!!
On a night flight, loose fitting t shirt and a pair of cheenos

nekkid

"What do people sleep in?" NOT a nightgown and bathrobe, not ever. I wear a comfortable outfit for the flight and sleep in it, something like a Soma-brand lounging outfit that doesn't look out of place in an airport. I assume this will be an overnight flight. Take a change of clothing and underwear in your carry-on, when you arrive in Seoul go to the Korean Airlines lounge and take a shower, change into the clothing you want to arrive in at your destination.

I've flown on many overnight flights and have yet to see anyone attired in actual jammies or a nightgown. Some airlines give out a PJ set, Virgin Australia comes to mind (at least they gave PJs a few years ago). Korean doesn't give PJs to business class passengers. Also, be sure to take a small flashlight so you can see around your seat in the dark without turning on overhead lights. You'll need to locate your shoes if you need to go to the lav while everyone's sleeping.