Simplest fix is tin foil.
Stick on with electrical tape. Easy to install and easy to remove and easy to replace. Not costly and you likely have a roll in the kitchen.
Then you easily try Joe's advice too ... now that we have daylight until midnight we're often staying up and spending time outside late in the evening. But we both like a dark room when we sleep so there are blackout curtains on the windows, which look like normal curtains but block light. In winter, they also provide a bit of insulation value on our drafty windows!