Could be a bad dream or just waking up and seeing shapes and thinking there are spiders. We went through this early this year with dd2, she has a night light/ torch so we used that to show her nothing was there. And again recently ds had a spider land in his hair in the car so he hated the car and going to bed for a few days, we had to hold him and show him there were no more spiders. I know others have made up special monster and bug spray which has worked too.