The Māṇḍūkya Upaniṣad maps consciousness into four states — three modern sleep science confirms, and a fourth beyond them.
Consciousness has four quarters: jāgrat (waking), svapna (dream), suṣupti (deep dreamless sleep), and turīya (the fourth).
— Māṇḍūkya Upaniṣad 3–7
Neuroscience distinguishes waking, REM (dreaming), and NREM deep sleep — each with its own brain-wave signature.
A genuine, defensible parallel.
The Māṇḍūkya Upaniṣad — among the shortest of the principal Upaniṣads — divides ordinary experience into waking, dreaming and deep dreamless sleep, and then points beyond them to turīya, pure awareness. The first three map remarkably onto what sleep science independently established millennia later: the waking EEG, REM sleep (vivid dreaming), and slow-wave NREM deep sleep, each a distinct neurophysiological state.
That correspondence is genuinely striking — careful introspection arriving at a three-fold division that electrodes would later confirm. The honest boundary is turīya: it is a contemplative, soteriological claim about the ground of awareness, and science neither confirms nor denies it — it simply lies outside what EEG measures. The first three are a parallel; the fourth is an invitation, not a data point.