If you can see a steady stream of wasps, circling around a particular area, then this is most likely the access point for their nest. My husband and I had noticed a few months ago that we were seeing more wasps than usual in our garden and every time we opened our wondows, we would get a few coming into the house. One particular day, when I was cleaning my son's bedroom window, I noticed several of them hovering around his window and as I watched them, I began to notice that they seemed to be arriving six or seven at a time and as they left, more were arriving. We called out pest control and the chap said that the nest was embedded within the guttering of the roof above our little boy's window (which is what we thought) and he stayed on ground level, hooked an extending tube onto the guttering and sprayed it and advised us to stay indoors for the next few hours, as they would become very aggressive. We couldn't believe just how many wasps were circling frantically after the nest was destroyed. It was truly terrifying to think of them swarming like that and we hadn't really noticed before. Thankfully our kids were never stung.
I would reckon that it would be unlikely that the wasps circling your bathroom window are doing so primarily for the source of light although this is more likely to draw them out as they are dormant during the night and avoid wet weather. You can either tackle the nest yourself (by spraying around where they are circling) if you purchase a pesticide, wear several layers of protective clothing and are confident that you do not have an allergy to wasp venom but personally, I would call out pest control. It's safer, easier and you are not risking your health by climbing up a ladder trying to exterminate a wasps nest because let's face it, they will be very aggressive when their nest is destroyed and it is best to be on ground level then trying to retreat down a ladder to safety. Good luck!