Sage Patanjali describes nidra (sleep) as a modification of the mind, where the mind rests or remains suspended in a state of absence of pratyaya (focus/object of meditation). The sutra distinguishes the state of sleep from the state of samadhi.

There is a story of Satyanand Paramhansa, founder of Bihar School of Yoga. He sat for dhyan in the evening and got up only in the morning, thinking he had achieved samadhi. His Guru, Shivanand, reminded him of a sutra from Rudrashtakam - 'Nidra samadhi sthiti.' The sthiti (state) of sleep is the state of samadhi only, but the avastha (condition) is not.

Sleep is a state of absence of pratyaya. Samadhi is that of 'viraam' pratyaya, when there is complete merger and as a result pratyaya ceases to exist.