[Advaita-l] Importance of Brahma muhurta - original source references

[Raghav Kumar Dwivedula]

Namaste
I request members to share any information regarded Brahma muhurta which
starts two muhurtas before sunrise and ends one muhurta before sunrise.
When we have a 6 am sunrise, this would mean the interval between 4:24 am
and 5:12 am.  This 48 minute interval is said to be ideal for dhyAnam etc.

Are there source references to this particular calculation of beginning two
muhurtas before sunrise etc., in texts? Googling gave me some generic info
about Agni and Siva puranas but nonspecific verses about brAhmI muhurta.

(Sri Ramakrishna mentions that the suShumna nADI is more active during this
muhurta.  and the breath tends to easily flow through both nostrils during
this.)

Lastly how does it work for the more northern latitudes where sunrise
occurs at crazy hours, by Indian/tropical standards.

Thank you

Om

Raghav