"Back in the day" I thought they only came on when the light faded. Mid 20s and very sunny? No way would the lights have been on.........

I've only been able to find basic info on floodlighting. Lights must be checked once a year by a qualified technician and a report of the findings sent to the EFL. 88 readings must be taken at pre determined places on the pitch and there are minimum requirements with regard to the lux readings. I couldn't find any mention of when they MUST be used.