You can. Years of sleeping on a saggy mattress may make the transition to a firm, supportive mattress feel like a shock. Give your body at least 30 days to adjust. Many mattress retailers understand that there is a breaking in period on a new mattress and offer no-strings returns after 30 days or sleep trials of up to 90 days. Read the fine print about return policies before purchasing so that you can give your body time to get used to your new firm mattress without worrying about getting your money back if it doesn’t work out between the two of you.