When you purchase a memory foam pillow online, it will generally be vacuum sealed to reduce the shipping size. Memory foam compresses under weight, which is why it’s so great for pressure relief, but it does take time to respond to changes in pressure. You’ll need to give your memory foam pillow a good 4 to 6 hours to reach 90% expansion and another few hours to expand fully. For the best results, give your pillow 24 hours to expand before sleeping on it.