Poorly designed book

I read to our son every night and recently, my wife pulled out a book called Five Shiny Stars. One day she told me it played music (there was a tag on the front of the book indicating it did), but I never heard it. So lately I’ve thought it was broken, so I banged the book against my head and magically it started playing “Twinkle, Twinkle Little Star”. Last night as I was putting Aiden to bed and reading the book, the son started playing when I turned to the last page (like it is supposed to do) without me banging my head against it. What was different about last night? It was still light outside when we put him to sleep and the sun was sneaking past the curtains. I looked closer at the book and show that there was a photo cell which triggers the song to start. This has to be the stupidest way to trigger music on a book that talks about going to sleep! Normally people read books about going to sleep when it is dark. I would have expected there to be a pressure type trigger to turn on the song.