In Śrīmad-Bhāgavatam, all kinds of siddhānta—established truths—are given. There, bhakti has been defined—lakṣaṇaḥ hi bhakti-yoga. Bhakti should be done for Kṛṣṇa without any cause, worldly cause, and it should be continuous, like an unbroken stream of honey. And that is why Rūpa Gosvāmī has defined bhakti with this verse:
anyābhilāṣitā-śūnyaṁ jñāna-karmādy-anāvṛtam
ānukūlyena kṛṣṇānu-śīlanaṁ bhaktir uttamā
I want you all to understand this verse. If you can know this, then you can follow bhakti. Otherwise, if you don’t know, then how can you follow? I want everyone to follow.
But what if we go to sleep or go driving and think about many other things? We should know all these things. If you are going to sleep, then you should think, “By sleeping I will be fresh and then I will serve Kṛṣṇa with a fresh mind and body.” Then it will be focused. We should sleep. Why? To be refreshed so that new energy will be given. Then we can continue the whole day and night. The whole life.
Srila Gurudeva: March 08, 2002 Kuala Lumpur