Sleep is the non-deliberate absence of thought-waves or knowledge [Iyengar]
Dreamless sleep (nidra) is the subtle thought pattern which has as its object an inertia, blankness, absence, or negation of the other thought patterns
Sleep is the vibrational mode which is supported by the absence of objective awareness.
While philosophers love contemplating the idea of dreaming, and how to distinguish it from being awake, the state of dreamless sleep is interesting, and less talked about. It’s true to say that while we are awake, we are aware that we are awake, but when we are in the state of dreamless sleep, we are not aware of this. Interpretations of this sutra, for example by BKS Iyengar, appear to suggest that during dreamless sleep, the mind is focused on the void or emptiness of sleep. I’m not sure that my use of the word ‘focused’ is quite right here, as this implies this is a conscious effort on the part of the mind, which is not what Iyengar is saying.
Iyengar goes on to say in his commentary that the Seeker is trying to achieve this void he has experienced while asleep, during his waking hours, to try to free the mind of sense experience. He says
Simulation of this state of sleep when one is awake and aware is samadhi [profound meditation], wherein the Seer witnesses his own form.
There is something quite Zen about all of this – The Rinzai Zen Buddhist koan the Gateless Gate states that mu (the negative, or void) is the gate to enlightenment. It is interesting how these two philosophies converge on this point. Sunyata is a key concept in Zen, the idea of emptiness. “Form is emptiness and emptiness is form”, as is chanted from the Heart Sutra. This appears to be a paradox – how can form possibly be emptines, and vice versa? There is a great explanation here, but in a nutshell ’emptiness’ does not mean ‘nothingness’ or having no form. These two opposing ideas of form and emptiness need each other to exist – without the notion of form, there can be no idea of emptiness (empty of what?). This is all rather mind-bending, and I do not claim to have got my head around it as of yet, it’s definitely a work in progress…