Chelseablue96; The short answer is that 'We simply Don't Know', to be perfectly honest! I don't think anyone's ever yet bothered to carry out a proper, scientific study of the phenomenon. Can't understand why not. Lack of available funding, maybe? Anyway, if I were asked to take an educated stab at it? First thing I'd suggest it the 'obvious' point that warmer air rises. So, they're up there because it's warmer and so they don't have to use up energy to keep warm ~ may seem rather negated by the fact that the idiots then use all that saved energy up by flying round pointlessly! LOL! Of course, flies flight Isn't pointless. They're doing their own things. Holding territory and hoping a mate will come along. Watch them and ye may notice how they 'bounce off of' nothing, in mid flight? That's their own little bit of space. So, Ok; We have them up there, because it's warmer. Hanging out like people in a bar. But, Why around the light fitting? Dunno. Maybe it's something to do with imperceptible ~ to us ~ air currents? That'd be my best guess. Maybe, to a flies on board navigational equipment, there's this swirling vortex of air movement around an object such as a light bulb? Work it out; Air's entering that room from somewhere. Air thus moves. Perhaps it moves in a spiral round the room as it warms and rises? If so, would there not be a still spot at its centre? " Eye of the Hurricane " sort of effect? Be more comfortable for a fly to fly there then, where it wouldn't be getting dragged by the swirling air moving round the rest of the room. And light fittings are normally central on our ceilings. Make sense? Know what? I reckon I might just have cracked it! Quite impressed myself there! Now I must go and lay down, because my brain hurts! LMAO!