A fitted sheet that pops off at night almost always has a pocket that is too short for the actual thickness of the mattress, mattress topper included. The three other possible causes are the wrong size, elastic weakened by the dryer, and fabric that has shrunk in the wash. A tape measure is all you need to find out which one applies to you.
What exactly is a fitted sheet?
The Office québécois de la langue française defines it as a "bottom sheet whose edges and corners, equipped with elastic, are designed so that the sheet conforms to the shape of the mattress." In France, it is called a "drap-housse." Both terms are correct.
It all comes down to the word conforms: a fitted sheet doesn't stay on thanks to the elastic alone. It stays on because its pocket goes down low enough for the elastic to tuck under the mattress. If the elastic stops on the side, the slightest movement will cause it to ride up.
The 4 causes, from most frequent to rarest
| Cause | The sign | The solution |
|---|---|---|
| 1. Pocket too short | The corner always pops off in the same spot, often at the head of the bed | A sheet with a pocket at least equal to the total thickness of the mattress |