Surah 78. An-Naba, Ayah 10

The darkness of the night is as a covering. Just as a covering protects us from exposure to cold or heat, so this covering gives us spiritual respite from the buffets of the material world, and from the tiring activities of our own inner exertions. The rest in sleep (in verse 9) is supplemented by the covering of the night with which we are provided by Allah.