The affix ghañ comes after the root sī 'to lie down' when the prepositions vi and upa are in composition with it and when the sense of the word so formed is that of 'regular succession'.,
As, तव विशायः 'thy turn of sleeping' (i.e. the rest enjoyed in rotation by the sentinels on watch); मम विशायः 'my turn of rest'; तव राजोपशायः राजानमुपशयितुम्पर्यायः 'thy turn of sleeping the king'.
Why do we say 'regular succession'? Observe उपशयः 'lying by the side of or ambush'; विशयः 'doubt'.,