Mandukya Karika 1.15
Sanskrit Verse
अन्यथा गृह्णतः स्वप्नो निद्रा तत्त्वमजानतः ।
विपर्यासे तयोः क्षीणे तुरीयं पदमश्नुते ॥ १५ ॥
Transliteration
anyathā gṛhṇataḥ svapno nidrā tattvamajānataḥ |
viparyāse tayoḥ kṣīṇe turīyaṃ padamaśnute || 15 ||
Translation
Svapna or dream is the wrong cognition of Reality. Nidrā or sleep is the state in which one does not know what Reality is. When the erroneous knowledge in these two disappears, Turīya is realized.
Related Upanishad Verse:

Muni's Play