For a yogi, restlessness is the night and restfulness is the day.

Verse 4.20

एक समये चोभयानवधारणम् ॥२०॥

eka-samaye ca-ubhaya-anavadhāraṇam ||20||

From its being unable to cognize two things at the same time.

It ( mind) cannot comprehend subject-object, observer-observed, or actor-witness at the same time, whereas the seer can.

Day and night cannot exist simultaneously. Similarly, restlessness and restfulness cannot co-exist in absolute juxtaposition. In between night and day there is dawn. In the same way, there is space between the flow of restlessness,

cittavrtti or and restfulness,. In

between these two rivers of restlessness and restfulness,.and underneath them, flows the concealed invisible secret river, the river of.the soul. This is dawn, or the sudden arrival of enlightenment.

For a yogi, restlessness is the night and restfulness is the day. In between, there is a third state which is neither day nor night but dawn. It is the diffusion of mind, in which the rivers of restlessness and restfulness unite in the seat of absolute consciousness.

When the water of a lake is unruffled, the reflection of the moon on its surface is very clear. Similarly, when the lake of consciousness is serene,

consciousness disseminates itself. This is known as a glimpse, or a reflection, of the soul.

The seer, being constant and unchangeable, can perceive the fluctuations as well as the serenity of his consciousness. If mind itself were self-luminous, it too could be the knower and the knowable. As it has not the power to be both, a wise yogi disciplines it, so that he may be alive to the light of the soul.

It is said in the Bhagavad Gita (11.69) 'One who is self-controlled is awake

when it appears night to all other beings, and what appears to him as night keeps others awake' . 

Similarly, in the Hatha Yoga Pradrpika, the word ha is used to signify the seer as the 'sun', which never fades; while tha represents mind as the 'moon', which eternally waxes and wanes.

PATANJALI YOGA SUTRAS