Look, we’re big fans of compression. The ability to take something as big and fluffy as a sleeping bag and store it in a small, portable bag is absolutely necessary for backpackers who need to carry it around on their back all day.
It is true that compression for an extended period of time can cause a significant reduction in loft, whether you have a down sleeping bag or a synthetic sleeping bag. Over time the air pockets in between the fill fibers become smaller and smaller. As the fill becomes more tightly compressed, the sleeping bag’s ability to retain heat becomes compromised.
While it is possible to return the sleeping bag to a fluffier, loftier state, it is possible for permanent damage to be done to a sleeping bag that is compressed for too long.
However, quick-compression periods of less than 24 hours (which is all you would need when you’re backpacking) do not damage the bag, especially when they’re allowed to puff back up overnight while you’re sleeping in it.