Valmiki Ramayana Yuddha Kanda Sarga 44 Shloka 1

तेषाम् their, वानररक्षसाम् Vanara and Rakshasa, युध्यतामेव battling, तदा then, रविः sun, अस्तम् set, गतः went, प्राणहारिणी harmful to the life, रात्रिः night, प्रवृत्ता began.

Thus, as the Vanaras and Rakshasas were battling, the Sun began to set, which is not favourable to the life of Vanaras. In the fierce fight that took place in the night Indrajith gets vanquished by Angada. Sri Rama and Lakshmana also put up a great fight and eventually get bound by a noose of serpents.