Based on the thoughts of various posters above, it's interesting to reflect that a policy (if indeed one officially exists) seen by some as intended to be a passenger 'benefit' might in fact revolve more around the convenience of crew, at least in its origins.

It's several years since I have flown long-haul with BA, so I cannot judge their current practice ; but there was a time when I was booking them monthly, mainly UK to Africa. I was never the best of sleepers on those trips (although it was before the days of full-length lie-flats & suites etc) and would often pass time chatting to FA's in the galley. Many would tell me that their priority on any night flight was to get the meal service done & dusted as swiftly as possible, dim all the lights, close all blinds, rack up the cabin temperature, and generally do everything they could to get as many pax as possible to nod off (or at least keep them quiet). The whole idea being - surprise surprise - to give the FA's themselves as peaceful and non-taxing a flight as possible.

Couldn't fault the strategy !!