After the rāsa dance was over, the night (the night of Brahmā, a very, very long period, as mentioned in the Bhagavad-gītā) turned into the brāhma-muhūrta. The brāhma-muhūrta takes place about one and a half hours before sunrise. It is recommended that one should rise from bed at that time and, after finishing daily ablutions, take to spiritual activities by performing maṅgala-ārati and chanting the Hare Kṛṣṇa mantra. This period is very convenient for the execution of spiritual activities.
Brahma-muhurta means
Srimad-Bhagavatam
SB Canto 3
The time early in the morning, one and a half hours before sunrise, is called brāhma-muhūrta.
SB 3.20.46, Purport: The time early in the morning, one and a half hours before sunrise, is called brāhma-muhūrta. During this brāhma-muhūrta, spiritual activities are recommended. Spiritual activities performed early in the morning have a greater effect than in any other part of the day.