I live in a small one bedroom apartment with the only window being in the bedroom. I've noticed that despite the AC cooling the rest of the apartment down, the bedroom is still warmer, and it's really affecting my sleep. I tried having the bedroom door open with a fan to better circulate the airflow, but it's not working, and I'm waking up quite hot. I realized that it must be the only window causing the problem, since glass is an awful insulator. Problem is, I'm not sure what to place on the window that I can remove during the day. It's a fairly tall window with one half of it being crankable, so I want to be able to open the window during the day. Black out curtains are a problem because I need the sun to wake up every morning (please don't suggest a SAD lamp). I mention that the window is tall because I can't hoist a big ass blanket over it every night with it being that tall.

What's something affordable that we can do for night time insulation? My sleep is really suffering.

PS: this place is rented, so I can't do anything too permanent.

TIA!