Robb Wolf suggested aluminum foil. Is that actually easy to put over a window? Does anyone have any less hideous solutions?
Film industry people use this to blackout windows to be able to shoot night interiors during the day (among other uses). It works!
Make sure you get the really thick kind.
I work nights so sleeping during the day is hard. I am planning on ordering these because they have gotten excellent reviews:
It is super simple and honestly, the most cost effective choice.
Yeah the eye mask does not = black out room as your skin is still absorbing it.
What I did is got a $1.00 long piece of PVC pipe from home depot, got some fabric ring clips, and folded 2 old sheets into a curtain. For under $20 I have probably a 90% blackout going (I'm still tweaking it) but it looks good and I can pull open the curtains during the day.
Just thought of something, a quick fix you can do in the meantime. If you have a large enough piece of cardboard (say, if one of your friends has just gotten some Ikea bookshelves) then you can cut it to fit your window just right so that the sides of the window make for slight compression in the cardboard, enough to keep it in place in the window. Then you also have something that is very easy to take down during the day and throw under your bed or wherever. I did this once when I was frequently sleeping in the middle of the day -- very un-paleo, I know. The trick is getting a big enough piece of cardboard.
I have blackout material lined curtains in my bedroom. Also extremely useful to have blackout blinds in young childrens rooms, especially in the summer when the light can otherwise wake them early. Doesn't feel very paleo...as it is artificially changing your light settings, but does allow you to sleep longer if that's your goal.
In the past, I also used black bin liners taped to my son's windows. Also looks tacky, but you could feasibly take these up and down pretty easily.
Our apartment complex has those lantern style floodlights and one was positioned such that it was always "sunny" in our bedroom. Actually, we were both quite sad since neither of us could sleep and the blinds plus "blackout" curtains didn't come close to addressing the problem.
Frustrated and needing a low-cost solution, did some shopping around and eventually found sheets of foam insulation (~ 1 inch thick with a foil coating on one side) at Lowe's and cut them to the size of our bedroom windows. I inserted them between the blinds and the window itself (with the white side facing out and it made a tremendous difference.
Its not a perfect solution (we leave them in all the time, so the windows stay closed), but for less than $20 it has done the trick.
I work 3rd shift, and sleep during the day, and the light coming in really bothered me. I stopped at the hardware/lumber store and bought "1/2-inch foil-sheathed foam", and cut it to JUST fit inside my window, and lightly tapped it into place. It blocks out all the light, and cuts out the drafts too. Bonus is that it keeps the cats and dog off my bed, since there's no view of the outside for them!
How to make your own blackout blinds from the blog Eat Sleep Move: http://eatsleepmove.com.au/how-to-make-your-own-blackout-blinds/