The new Hibermate uses 4 layers of material to prevent light seepage. The first is the outer satin cover, the second is a layer of firm inner material to help the mask maintain it's shape, and the side which rests against your skin is a wonderfully soft cotton jersey. Sandwiched between is a layer of wadding (or batting), which is used as padding, but it too helps prevent light getting through to your eyes.
The way the mask has been designed, prevents any light getting in from the sides. You may experience a tiny amount of light seepage from under the bridge of your nose, and if so, simply put your finger on the bridge of your nose and move it upwards - this pushes that part of the mask into a snugger position and helps it fit better on your face.