Ya nisa sarva-bhutanam tasyam jagarti samyami
yasyam jagrati bhutani sa nisa pasyato muneh
(Text 69, Contents of the Gita Summarized)
Meaning: What is night for all beings is the time of awakening for the Self controlled; and the time of awakening for all beings is night for the Introspective sage.
There are two classes of intelligent men.One is intelligent in material activities for sense gratification, and the other is Introspective and awake to the cultivation of self realization. Activities of the Introspective sage, or thoughtful man, are night for persons materially absorbed. The sage feels transcendental pleasure in the gradual advancement of spirtual culture, whereas the man in materialistic activities, being asleep to Self-realization, dreams of varieties of sense pleasure, feeling sometimes happy and sometimes distress in his sleeping condition.The Introspective man is always indifferent to materialistic happiness and distress.He goes on with his self realization activities undisturbed by material reactions.