Perplexing is a verb
This means to disturb mentally; confuse
Merriam- Webster’s dictionary
“The Yellow Wallpaper” by Charlotte Gillman
“I’m feeling ever so much better! I don’t sleep much at night, for it is so interesting to watch developments; but I sleep a good deal in the daytime. In the daytime it is tiresome and perplexing.”
I see how this word fits into the context. During the daytime she’s more mentally disturbed which makes it more easier to sleep during the day.