Most people tend to forget that sun light can do more then just light a space, but it can also heat it. When light particles travels and converts kinetic energy, part of it gets transformed into waste heat energy, this is what we can use to heat our homes and spaces. So open your windows in during the day in the winter and help cut down your heating costs, and your electricity consumption.
But when the sun sets and night comes, it's important to close those blinds. This is because heat escapes at night. When there is a state with greater and less heat energy, a gradient is created because the side with more will always try to spread out to the lesser. It's like osmosis but with heat. Hence by closing the blinds you're creating an extra barrier for the heat to escape, as well as providing a surface for the light in your room to bounce off of to continue offering it's heat energy to your room.
Also note that when light passes through glass, it increases the amount of heat exhaust produced, thus more glass in the room, the more free heat you'll have.
