The term yoga-nidra has been used in many empirical studies to refer to relaxation and guided imagery. These techniques do not represent the intention or physiological correlates of yoganidra discussed in the traditional yoga literature. We propose an operational definition of yoga-nidra that is supported by several physiologically testable hypotheses regarding its outcomes and effects. Traditional descriptions of yoga-nidra and contemporary accounts of its practice are reviewed, and studies examining the physiological correlates of yoga-nidra are examined. Proposed hypotheses for future research using this operational definition are provided.