It sure seems like leaving the curtains (or shades) open when it's sunny,
and closing them when it's not sunny does about the same thing.
Passive solar heating. I've been doing since 1973..
At this very moment, I have the very bright 8AM sun coming in the windows of my PC room. (Toasty)!
It's saving me heating oil and helping with the S.A.D. at the same time!
Cheers,
Rich