Brihat Parashara Hora Shastra
सन्ध्याद्वयञ्च विंशत्या नाडिकाभिः प्रकीर्तितम् । दिनस्य विंशतिर्घट्यः पूर्णसंज्ञा उदाहृताः ॥ ४५॥
The total period of both these Sandhyas (twilight) is said to be 20 Ghatikas. The 20 Ghatikas of the night have been given the name Purna and the 20 Ghatikas of the day have been given the name Mugdha.
english translation
sandhyAdvayaJca viMzatyA nADikAbhiH prakIrtitam । dinasya viMzatirghaTyaH pUrNasaMjJA udAhRtAH ॥ 45॥
hk transliteration by Sanscript