svapna dream state, a state of delusion
nidra sleep state
jnana wakeful state, awareness, intelligent state
alambanam support, base, dependence or resting upon, assistance, help, distinguishing the gross from the eternal
Or, by recollecting and contemplating the experiences of dream-filled or dreamless sleep during a watchful, waking state.
Citta has four planes - the unconscious, subconscious, conscious and super-conscious. The unconscious plane is the state of dreamless sleep (nidra). The subconscious plane is the dream-filled (svapna) state. The conscious plane is the waking (jagrata) state. The super-conscious plane is the fourth state known as turya. Turya is samadhi, the ultimate state where the individual soul (jivatman) is united with the Universal Soul (Paramatman).
By close scrutiny of dream-filled and dreamless sleep, the sadhaka comes to discern the various levels of consciousness, and learns to convert them into a single state of consciousness.
The sadhaka should also ponder on the thought of the soul before going to sleep, so that the same thought flows uninterruptedly whether he is awake, dreaming or asleep. This supports advancement towards the attainment of spiritual bliss.
In m.ii-12 Patanjali explains ksaya (waning) citta, santa (calm) citta and udaya (rising) citta. These may be equated with svapna, nidra and jagrata states. By and large, declining thoughts lead to quietude, but strong rising thoughts keeps one wakeful. A yogi maintains passive alertness without allowing thoughts to spring forth, or endeavours to hold them back. This is reflective contemplation.