Yoga Nidra is a style of yoga which emphasizes reaching a deeply meditative state through gentle breathing and visualization exercises. The word nidra in fact means sleep, and state of meditation is described as a waking sleep, in which you are conscious but completely resting in body and mind. The result for most students is attaining a level of relaxation and ease they've never felt.
Here is a simple 10-minute Yoga Nidra exercise in which you move your attention to different spaces of the body in a repeating pattern. You'll begin lying down in a comfortable position, with your head and limbs supported in a way that there is as little strain on your body as possible. (You can find ideas for a comfortable savasana pose here.)