Sleeper coaches on meter gauge are common in South East Asia.
Before India converted its entire network to broad gauge, there were meter gauge sleeper coaches in mainlines.
Japan + parts of Africa use cape gauge, which is only slightly wider (1067 mm) and there are plenty of sleeper coaches in this gauge.
Agree that it is rare in Europe due to smaller sizes of countries.
I measured with Playmobil figure lying on floor, in a Playmobil panorama coach, there is enough space to create sleepers.
To create a realistic sleeper coach, one needs a long 2x2 axles coaches like Panorama coach.
Layout can be of 2 types
[1] traditional sleeper with corridor on one side
[2] business jet type seating converted to beds - similar to Panorama coach layout. The table goes down and seats fold to make bed
In this hot weather we can close the curtains and pretend it is night time and run sleeper coaches indoor