Potentially one of my lifetime best investments. Not just a much better night's sleep, but also great for being able to nap/chill during the day or watch a movie (especially if you have a projector setup).
While evaluating and installing better options from e.g. blinds.com or Home Depot. I used cellular blackout blinds and opted to get a system where they can be lowered from the top as well as raised from the bottom — I highly recommend that option in order to be able to bring light into your space while maintaining privacy or perhaps minimizing your own distractions.
I've often used temporary paper substitutes which are maybe even inconveniently durable (it took a long time for me to pull the trigger on my first set of real blackout blinds 😬)
Fun fact: in Germany, every window has a metal blind that rolls down the exterior of the building, blocking out all light. It's so convenient. (Generally their overall window/door tech is quite impressive).