Days end with the sun's setting and Nights with the sun's rising. The end of pleasure is always sorrow, and the end of sorrow is always pleasure.
अहान्यस्तमयान्तानि उदयान्ता च शर्वरी। सुखस्यान्तं सदा दुःखं दुःखस्यान्तं सदा सुखम्॥१८