Bhagavad Gita · Chapter 14: The Three Qualities of Nature
Bhagavad Gita 14.11
सर्वद्वारेषु देहेऽस्मिन्प्रकाश उपजायते ।
ज्ञानं यदा तदा विद्याद्विवृद्धं सत्त्वमित्युत ॥
sarvadvāreṣu dehe'sminprakāśa upajāyate | jñānaṃ yadā tadā vidyādvivṛddhaṃ sattvamityuta ||
When the light of knowledge shines through all the gates of this body, then one should know that clarity has grown strong.
Modern context
How do you know sattva is strong? Light shines through all the gates of the body: the eyes see clearly, the ears hear well, the mind understands easily. Everyone has had such days, when everything is vivid and makes sense. Those are times to do important thinking and make important decisions.