strange idle

[stevie-g1968]

over the winter ive been tinkering ( read cocking up ) and tidying up my bike.
I had it running in the garage after a little run out this morning.
while ticking over the idle sits at about 1150 to 1200 rpm with the lights on... when I turned the lights off the idle went up to around 1600 rpm... so the burning question is why?, coz ive not got a clue..

[Paulfzs]

ive had many bikes that do that, never been a problem really.