Brahma Muhurta:

A) Muhurta (Sanskrit: मुहूर्त) is a Hindu unit of measurement for time in the Hindu calendar . A Muhurta equals 2 Ghadiyas or 48 minutes.

BRAHMA MUHURTA starts 96 minutes before sunrise. It lasts for 48 minutes. Brahma Muhurta ends 48 minutes before sunrise.

From 04:24 AM to 05:11 AM is BRAHMA MUHURTA (Sunrise is taken at 6 AM).

During this Brahma Muhurta, spiritual activities are recommended. Spiritual activities performed early in the morning have a greater effect than in any other part of the day.

1 Muhurta = 48 minutes. 1 day = 24 hrs = 30 Muhurtas.
Brahma-Muhurta is the 2nd last Muhurta before sunrise. (1 hr 36 min
before)

The Time of Rising from Bed:

"The time early in the morning, one and a half hours before sunrise,
is called brahma-muhurta. During this brahma-muhurta, spiritual
activities are recommended. Spiritual activities performed early in
the morning have a greater effect than in any other part of the day."

Ref. VedaBase => SPL 48: At Home in India: (In Vrindavana)

"The exact time for the mangala-arati would vary according to
the clock, he (Prabhupada) said, but it must always be one-and-a-half
hours before sunrise."

The last line of Shri Gurvastakam appears in the Stavamrta-lahari of
Shri Visvanath Chakravarti Thakura as follows:

shrimad-guror astakam etad uccair brahme muhurte pathati prayatnat
yastena vrndavana natha saksat sevaiva labhya januso 'nta eva (9)

"That person who very attentively recites this astakam to Shri Gurudeva
during brahma muhurta is sure to achieve direct service to
the lotus feet of Lord Shri Krishna, the very life and soul of
Vrindavana, at the end of his life."

Shrila Prabhupada writes in Cc. Madhya 24.331, purport:

"In the early morning hours (known as brahma-muhurta) one
should get up and immediately chant the Hare Krishna mantra, or
at least `Krishna, Krishna, Krishna.' In this way, one should remember
Krishna."

Krishna book, "Lord Krishna's Daily Activities":

Lord Krishna would immediately get up from bed exactly on the
appearance of brahma-muhurta.. After rising from bed, Lord Krishna
would wash His mouth, hands, and feet and would immediately sit down
and meditate on Himself.