Complex sentence: The first "prahara" of the day is widely thought to commence with the brahma-muhurta, a period of time before sunrise, but the commonly available secondary sources contradict each other as to when exactly the "brahma-muhurta" begins.
Split into simpler sentences: The first "prahara" of the day is widely thought to commence with the brahma-muhurta, a period of time before sunrise, but the exact time when the "brahma-muhurta" begins is disputed. The commonly available secondary sources contradict each other as to exactly when the "brahma-muhurta" begins.