Bhagavad Gita · Sankhya YogaChapter 2, Shloka 69 (2.69)

Reversed day, reversed night. This is one of the most poetic shlokas of the Gita. The same "time" — day and night are reversed for two kinds of people.

Shankaracharya / Madhusudana (the Advaita reading):

• The night of all beings = paramārtha-tattvam — the highest truth, which, being beyond the reach of the understanding of the ignorant, is like darkness for them. The self-controlled one is awake in it — awakened from the sleep of ignorance.
• That in which beings are awake = grāhya-grāhaka-bheda-lakṣaṇā avidyā-niśā — the night of ignorance marked by the division between seer and seen; beings are said to be "awake" in it even while asleep, svapna-dṛśaḥ — like people seeing a dream. For the one who sees the truth, it is night.
• Shankaracharya's comparison: just as what is day for naktañcara (creatures that roam at night) is night for others. Madhusudana gives a shloka of the Varttikakara — kākolūka-niśevāyaṃ saṃsāraḥ — the crow is blind at night, the owl is blind by day; one's day is the other's night.

Ramanujacharya (the Vishishtadvaita reading):

• The night of all beings = ātma-viṣayā buddhiḥ — the understanding concerned with the self, which gives no light to ordinary people. The self-controlled one is awake in it — ātmānam avalokayan āste — he remains seeing the self.
• That in which beings are awake = śabdādi-viṣayā buddhiḥ — the understanding concerned with objects such as sound; for the muni who sees the self, it is like night — it gives no light.