Al-Isra · 17:12

12. I have created the night and the day as two signs indicating Allah’s oneness and power, due to their differences in terms of being long or short, hot or cold. I made the night dark for the purpose of rest and sleep and I made the day bright in which people can see and work for their livelihood. This was so that by the alternation of day and night you can know the number of years and calculate the months, days and hours that you require. I have explained everything in detail so that it may all become clear and the truthful can become distinguished from the liars.