Deep sleep is that fluctuation of the mind which rests on the non-existence of objects.

Why Sleep Counts as a Fluctuation

Nidrā is defined as a vṛtti whose ālambana, its support, is the pratyaya of abhāva, the cognition of absence itself. Vyāsa's argument for treating dreamless sleep this way rather than as a simple mental blank rests on a familiar experience: a person who wakes and says 'I slept deeply and was aware of nothing' is reporting something, which implies a modification of the citta occurred during that sleep for memory to later retrieve. If the mind had truly switched off, there would be no experience left to recall, and the report itself would be impossible.

Tamas as the Operative Guṇa

Vācaspati Miśra locates nidrā under the dominance of tamas guṇa, distinct from the states of sattva-dominant clarity that meditative absorption seeks, even though both can look externally similar as stillness from the outside. This distinction is not cosmetic. Tamas brings heaviness, dullness, and a withdrawal from discrimination, while the clarity aimed at in samādhi is alert, luminous, and highly discriminating even in its stillest form. Treating the two as equivalent because both involve outward inactivity misses the qualitative difference the tradition insists on.

The Sutra as a Boundary Marker

By giving sleep its own definition here, Patañjali closes off a possible misunderstanding that could otherwise arise from the earlier discussion of nirodha, the quieting of mental fluctuations. A student might assume that any cessation of active thought, including unconsciousness, is progress toward that quieting. This sutra rules that out directly: nidrā is itself a vṛtti requiring the same eventual understanding and mastery as the other four, not a shortcut past them. Recognizing sleep as an object of study rather than a void to fall into is part of extending yogic awareness across the full range of the citta's activity, including the hours ordinarily treated as simply lost.

Observing the Threshold Into Sleep

This practice brings a thread of awareness to the transition most people cross unconsciously every night.

The Falling-Asleep Witness

1. As you lie down, resolve in advance to notice the transition rather than simply drift off.
2. Observe the body growing heavy as tamas rises.
3. Notice thoughts becoming fragmented and less coherent.
4. Try to enter sleep as a witness rather than being pulled under by it.
5. Hold a single quiet intention, such as noticing the absence itself, as your last conscious effort.

Extending Awareness Into Waking

6. Upon waking, before moving, notice the first sensation that returns.
7. Ask whether any trace of the 'nothingness' object of sleep is still present.
8. Notice the exact moment ordinary thought resumes, naming it as memory, concept, or perception.
9. Record any difference in clarity between mornings practiced this way and mornings when the exercise was skipped.

Equating Deep Sleep With Meditative Absorption

A common error is prizing dreamless sleep as spiritually equivalent to meditative stillness because both feel restful and free of active content. Vyāsa's tamas-versus-sattva distinction rules this out directly: deeper absorption is built on heightened, luminous alertness, while sleep is built on the heaviness of tamas overtaking the mind. Seeking drowsiness during seated practice mistakes a sinking mind for progress when it is instead a departure from the very alertness the path requires.

Assuming Sleep Is a True Gap in Experience

Many assume sleep is simply a hole in consciousness rather than an experience with its own content. If it were a true gap, the later report 'I slept and knew nothing' would have nothing to draw on; the fact that such a report is possible shows a modification, however faint, occurred throughout.