Dozy buzzing flies coming out of the woodwork

[RJUKL]

I am lying in bed reading when I think I hear the distant sound of buzzing. I pause and listen carefully. Nothing! This happen time and again. Eventually it gets louder and I hear/feel a faint plop in my pillow whereupon I leap up and try chasing it with anything to hand. It eventually goes to ground without being zapped. After several of these disturbances I go downstairs and bring back some fly spray and lay in wait. I hear nothing so begin reading again and it starts all over again. Is this a common occurance at this time of year?