A room used only for sleeping, kept quiet and dark, is a historically recent arrangement. It became possible when housing space and heating stopped being scarce enough to force sharing.
Rooms were multi-purpose because heat was expensive
In houses heated by one fire, the heated room was where the household lived, ate and often slept, because heating additional rooms was not practical.
Beds appeared in rooms that served other functions during the day, and furniture that folded or converted was common for exactly this reason.
Central heating removed the constraint. Once every room could be warm, activities could be distributed and a room could be reserved for one use.
Space per person rose steadily
Average dwelling size relative to household size has increased substantially over the last century in most industrialised countries.
Additional rooms are allocated to sleeping before almost anything else, which is why room counts are the standard way of describing a property.
Shared sleeping between siblings, and between generations, declined as a consequence of the additional space rather than as a change in preference.
The single-purpose room changed expectations
A room used only at night can be arranged entirely around night conditions: heavier curtains, softer light, fewer surfaces that need attention.
Those arrangements then become the standard, and disturbance that would have been unremarkable in a shared living space becomes something the household works to eliminate.
The expectation is a product of the arrangement. People sleeping in multi-purpose rooms are not tolerating a worse version of the same thing; they are operating under different norms.
Building standards followed the practice
Regulations covering sound transmission between dwellings, window sizes and ventilation were written with the separated sleeping room assumed.
Developers design to those standards, which reproduces the layout in new housing regardless of how any particular household would prefer to live.
Older housing stock converted into flats frequently sits awkwardly against this, since rooms designed as parlours become bedrooms with the acoustics of a parlour.
Devices reversed part of the separation
The single-purpose bedroom was achieved and then partly undone, as screens brought work, television and conversation back into the room.
The functions that were distributed to other rooms over a century returned within a couple of decades, without the furniture that once marked them.
Advice to keep the bedroom for sleeping is therefore an attempt to preserve a fairly recent arrangement, not a return to how people traditionally lived.