Yoga Nidra is a meditative state between sleeping and wakefulness. It promotes a really profound state of relaxation.
Here is a podcast with Richard Miller who is a teacher and scholar on Yoga Therapy and has developed a Yoga Nidra protocol (on iRest). I recommend it, it’s really well done.
Here is a link to an audio-recording of a short Yoga Nidra practice that you can try.
Here are other teachers who specialize in this practice: