"The propensity which creates a psychic vacuum is called nidra´ (sleep). Sleep is the intervening stage between two periods of active psychic functioning. It is a stage of vacuity in which the mind does not function. On waking up one makes the safe assumption that one’s mind did not function for a certain period – that one slept. Klis´t´a nidra´ (“painful” sleep) is an impediment to sa´dhana´. But dreamy sleep and deep sleep in which one is aware of one’s sa´dhana´ is certainly not detrimental to spiritual progress. This sleep can be called aklis´t´a nidra´ or non-painful sleep. Sometimes during sa´dhana´ one falls into a relaxed “half-sleep”. This is called yoga nidra´. Most sa´dhakas experience yoga nidra´ in the early stages of sa´dhana´. It is also aklis´t´a because one does not become totally oblivious of oneself."
Shrii Shrii A'nandamu'rti
A’nanda Marga Ideology and Way of Life in a Nutshell Part 9