Abhaav pratyayah alambana vrittirnidra
Sage Patanjali goes on to describe 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 assumes great importance in the light of distinguishing the state of sleep from the state of samadhi because otherwise the eight to ten hours that we sleep every day would also be yoga.
There is a story of Satyanand Paramhansa, the founder of Bihar School of Yoga. Satyanand used to sit for dhyan in the evening, and once, in the initial years of his yog sadhna, he kept sitting till morning. In the morning when he opened his eyes, he ran straight to his Guru, Shivanand, and told him, "You know what? I have achieved samadhi today." He was then asked, "How is that?" He replied, "I sat in the evening for dhyan and got up only in the morning. I did not realise the time at all." So Shivanand told him that it is not samadhi he had achieved, but that he had fallen asleep. He reminded Satyanand of a small sutra from Rudrashtakam - 'Nidra samadhi sthiti.' The sthiti (state) of sleep is the state of samadhi only, but the avastha (condition) is not. The avastha is different.
Sleep is a state of absence of pratyaya. Samadhi is that of
'viraam' pratyaya (Samadhi paad, sutra 18), when there is complete merger and as a result pratyaya ceases to exist.