Explanation

‘Ā'ishah (may Allah be pleased with her) says that the Prophet (may Allah's peace and blessings be upon him) used to sleep at the beginning of night, after praying the ‘Ishā' and would pray Qiyām at the end of the night, which is the second third of night. After finishing his prayer, he used to return to his bed and sleep, and that would be at the last sixth of the night to take some rest after the hard Qiyām. This sleep is also useful to restore one's energy for the Fajr prayer and the morning supplications. With this sleep, one wakes up refreshed. No one will discern that he was doing extra worship at night. Thus, ostentation on part of the worshiper would be out of the question. For this reason, the first call to the Fajr prayer is meant to wake up those who are asleep to pray the Witr if they have not done so and to urge those who are praying to take some rest in order to restore their energy.