Pistons (Sticky or otherwise) are behaving as completely transparent blocks. They are not blocking light traveling through them. This includes both daylight and block light such as torches and lava. I have attached screenshots of the same room at noon, midnight, and at noon with the pistons replaced with solid blocks. As per usual, I apologize in advance if this has been reported already as I could not find it.

Edit:
I know that pistons are transparent blocks, but for some reason I remember them not having that specific property thereof. I went back to an older version and they still transmit light. My apologies for the bug report.