Yoga Nidra translates as "yogic sleep". It is a state of deep relaxation in which the body is asleep and the mind is in a calm state of awareness. While the practice is intended for conscious relaxation it is fine to use this guided practice to help you relax and drift into a deep state of sleep for both body and mind.
If you fall asleep every time you practice - don't worry - it simply means you need sleep. When you come back to conscious awareness, simply feel the body resting and supported and feel the movement of breath. With regular practice you will become rested enough to stay awake mentally but allow the body to sleep and enter this state of open awareness.
This practice is audio only.