Kimono?? That's Japanese. This is a Korean manhwa -__- Yeha
Ohh.. So thats why. I dont watch korean dramas cause it always make me cringe. Its just so cringey. I didnt know that historical people wears pajamas too. Wow.. pajamas might have been worn since the dawn of time.
Its just weird that he wears pajamas to sleep. I mean this setting is from a really long time ago. A kimono would be more logical or a white robe.