If you're used to a traditional bed frame, a platform bed may seem a bit too low for two reasons: The bed base itself is lower than your usual bed frame, and there's no box spring beneath the mattress. Add height to your platform bed with bed risers, by adding a box spring, or by making your own risers for each bed leg or the entire bed.
Bed risers or bed lifts add 3 or more inches of height to a platform bed that rests atop four legs, much like a regular bed. Some of the risers look a bit like upturned flowerpots with an indentation in the top; others have a wood finish. They're sold in a variety of heights and styles; just be sure to measure the width of your bed legs to ensure they'll fit inside the indentation on top of the risers. Place one riser under each bed leg, making sure the legs are positioned securely atop the risers before you sit or rest on the bed. Enlist several friends to help so you can place risers under two legs at a time for less stress on the bed's legs.
Box Spring Booster
If using a traditional mattress atop your platform bed, dig the matching box spring out of the closet and set it atop the platform bed, followed by the mattress. Purchase a new box spring compatible with your mattress size if you don't have a box spring handy. The sleeping experience should feel similar to sleeping on the mattress without a box spring, although the mattress may feel less firm. Do not use an old wobbly or broken box spring atop your platform bed, as the mattress may sag in some areas, making the bed uncomfortable.
Build Your Own Solution
For a bed with four legs, build your own risers out of scrap wood. Stack and glue several boards together to the desired height; then build a shallow frame or "walls" around the top level to secure each leg in place. If your bed frame continues around the entire base of the bed instead of using legs, create a stage-style platform out of new and reclaimed untreated lumber. Give your bed a music-inspired upgrade by using stage or drum risers to lift and support the entire bed safely without woodworking tools.