They are twin filament 12 volt 21/5 watt bulbs, as used in stop/tail lights. 21 watts used during the day & 5 watts used at night (side lights)Does it happen for other people? I assumed it was because during the daytime you want *brighter* lights, or no one would be able to even see them. At dusk, when you'd normally have side-lights on - they don't need to be so bright? Who knows. I had a look and they seemed to be the same bulbs, but I'm not sure, didnt' check properly.