It might be an allergy or it might just be a bit of fluff from feathers etc...
Strictly in the spirit of scientific enquiry, of course, try a session of fly-tying during the day to see if the problem recurs. Preferably after running the vacuum to remove lingering allergens. You might be allergic to some pollen or other. IIRC, tree pollen generally releases at night and grass flowers generally release pollen during the day. So if you start to suffer from hay fever at night, especially when away from your tying bench it might be tree pollen. Your local TV weather report or newspaper might carry a pollen report.
Regardless. I think you need to do some more fly tying, strictly in the spirit of scientific enquiry, of course.


Regards,
Ed