The sun can really heat up a room quickly. If you are able to block the sunlight from entering your home you can help keep the temperature much cooler. If you can afford them, you can buy insulated shades that absorb the heat. I have them on one of my south facing windows and they are wonderful. This is my first summer with them and I think I may need to purchase some more. They do a really good job.
And if you are looking to still let light in but limit the amount of heat, then it is important that you close the shades on the windows to the south and west in your home (at least in the area of the world that we live in.) Light still enter in through the north and east windows but they won't be in direct sunlight during the hottest part of the day, which is when you really want to have those shades drawn.