Yoga-nidra is not nidra

Yogis are advised to keep their eyes half closed, but this state is not at all one of sleep, although imitation yogis, especially in the modern age, manifest their so-called yoga by sleeping. In the shastra, yoga is described as dhyanavasthita, a state of full meditation, but this is meditation upon the Supreme Personality of Godhead. Dhyanavasthita-tad-gatena manasa: the mind should always be situated at the lotus feet of the Lord. Yoga practice does not mean sleeping. The mind should always be actively fixed at the lotus feet of the Lord. Then one’s practice of yoga will be successful.

Srimad Bhagavatam 7.9.32 purport