Yoga Nidra comes from an ancient Tantric tradition for cultivating a mental sense of quietude and deep physical and emotional relaxation. Nidra, in Sanskrit, means sleep and the state of Yoga Nidra is one of conscious rest, whereby the practitioner sustains the liminal state between sleep and wakefulness for the duration of the practice. In sleep medicine this state, predominated by alpha brain waves, is called the hypnogogic state and has been shown useful for inducing a calm and restful mind.