Does 20 minutes of yoga nidra really equal hours of sleep?

That claim is overstated, and it does not trace to a study. Yoga nidra is deeply restful and can leave you genuinely restored, but it does not replace the biological work your body does during real sleep. When researchers put practitioners on EEG during yoga nidra, every recording scored as awake, with sleep-like slow waves appearing only in some regions of the brain. Whether that reduces sleep debt is explicitly unknown. Treat it as excellent rest, which is valuable on its own terms, rather than as hours back in the bank.