.... Most beds for royalty in prerevolutionary times (before the revolution of 1917 in Russia) were much shorter than the height of their owners. The reason was that doctors supposed it was much more useful for people's blood circulation and breath to sleep half-sitting/half-seated....
I usually say "half-seated", but I'd like to know whether the second option is also possible.
Can we say "half-lying" or "half-lied" with the same meaning or these words don't exist?
He was seated on the chair = He was sitting on the chair
He was lied on the bed is incorrect and should be "He was lying on the bed".
You can describe someone as "half-seated and half-lying" on something but "half-sitting and half-lying" sounds better.
Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.