Surya Siddhanta
अयनान्ते विलोमेन देवासुरविभागयोः। नाडीषष्ट्या सकृद् अहर्निशाप्य् अस्मिन् सकृत् तथा ॥
There occurs once, at the end of the sun's half-revolution from solstice to solstice, a day of sixty nadis, and a night of the same length, mutually opposed to one another, in the two hemispheres of the gods and of the demons.
english translation