Śivasaṃhitā 3.27

Time is not neutral in classical yoga. The four junctions prescribed by this verse — dawn, noon, twilight, midnight — correspond to sandhyā, moments of cosmic transition when opposing principles briefly balance. Practicing at these thresholds amplifies the efficacy of each kumbhaka, as the autonomic nervous system undergoes its own cyclical changes at those precise instants.

Prātaḥkāla (dawn time) and arddharātraka (midnight) are the daily cycle’s extremes; madhyāhna (noon) and sūryāsta (sunset) the midpoints. All four form the caturyāma — the four watches — of daily time. This fourfold division replicates in miniature the yuguic cosmology: four eras, four moments, one single principle of equilibrium and transformation.

Four-times-daily practice was standard in medieval tantrasādhana traditions. The Indian monk was to rise during brāhma muhūrta — before dawn — for the first session. The midnight practice, most powerful according to the texts, required extraordinary discipline that few maintained, which explains the extraordinary promises reserved for the most consistent practitioners.