Retired US Navy pilot
They don’t. They sleep on a futon mattress that is on the floor.
I lived in Japan for 3 yrs.
They sleep on a futon, a kind of heavy foam mattress, that lies on top of a straw mat, called a tatami mat. The futon is laid on the floor. You never walk into a tatami mat room with your shoes on, and usually without sleepers. The floor of the entire room is a tatmi mat.
It is like the tatami mat is the box springs for a bed, and the futon is the foam mattress.
But they do not use the kind of bed frx and headboard that you see in the US and other Western countries.
And I can say it’s quite comfortable.

美国退役海军飞行员他们不这样。他们睡在地板上的榻榻米床垫上。
我在日本住了3年。
他们睡在一种重型泡沫床垫上,这种床垫放在一种稻草垫子上,叫做榻榻米垫子。床垫放在地板上。你绝不会穿着鞋子走进榻榻米房间,通常也不会穿拖鞋。整个房间的地板都是榻榻米垫子。
就像榻榻米垫子是床的弹簧床垫,而床垫是泡沫床垫一样。
但他们不使用美国和其他西方国家所见的那种床架和床头板。
我可以说这样睡觉很舒适。