Saving power - off/on/off/on

Has anyone some expertise about power saving from devices — motion sensors? — which automatically switch-off lights when there's no activity in a room?

I can see the point of installing them, when I see office buildings at night blazing with light at times when it seems unlikely people are still working - though of course, the cleaners may be. On the other hand there are places like libraries where putting in these systems is frankly bonkers. It's essential for public confidence and basic safety purposes that such public spaces are well-lit and enable overlooking.

I thought about the question yesterday at a meeting in Haringey Civic Centre. Seven people sitting talking and leaning over a map wasn't enough to stop the lights constantly going out. So we needed to walk around and flap our arms.

Does the frequent switching on and off even save any energy?