The four states

Two thousand years ago, a very short text had already drawn a map of consciousness. The Māṇḍūkya Upaniṣad, twelve verses in all, says the mind has not two states but four, and ties them to the three parts of the sound Auṃ plus the silence that follows: waking (jāgrat), dream (svapna), deep dreamless sleep (suṣupti) and a fourth, turīya.

The surprising one is the third. To common sense, deep sleep is the dark, the nothing. The tradition says the opposite: suṣupti is not a void but a full, undivided state of peace. Not that nothing happens there; it is that we bring nothing back. Consciousness, the text says, never fully switches off: it changes shape, and sometimes simply leaves no trace.

This is exactly the question science still asks today. For a long time deep sleep was treated as consciousness switched off. But when people are woken from any stage and asked, some report an experience even from deep sleep, and some say: I was experiencing something, but I cannot say what. Whether consciousness truly disappears in dreamless sleep, or only becomes unreportable, remains an open question (Windt, Nielsen and Thompson, 2016).

And dreaming does not live in REM sleep alone. Siclari and colleagues, waking sleepers hundreds of times, found that dream experience tracks a posterior hot zone of the brain and can appear even in deep sleep. It is not the sleep stage that marks consciousness, but the presence or absence of experience (Siclari and colleagues, 2017). Again: not a switch, but a texture that shifts.

The four are not steps of a staircase descending into nothing. The fourth runs beneath all the others, like the thread on which the beads are strung. Waking is not the top and deep sleep the bottom: they are forms, and something continuous holds them together.

And the road? On the road we use only two words, awake and asleep, as if a clean line ran between them. It does not. Between waking and sleep there is a real, inhabited zone - drowsiness, the first slide of local sleep - and the map of the four states is honest where our folk model is not. The danger is not falling asleep at the wheel: it is believing yourself fully awake while you are already on the threshold.

Before setting off, it is worth remembering that between the waking and the sleeping there is no door, but a threshold.