Yoga Nidra or Non-sleep deep rest (NSDR), is the kind of rest when the body is in a relaxed state, but the mind is still awake.

It usually involves following guided meditation that helps to slow down the brain wave frequency, similar to what happens during sleep, only that in this case person is awake.

Sources:

https://www.youtube.com/@hubermanlab

https://www.joinvitalhealth.com/blog/non-sleep-deep-rest