Brachos 4 – ברכות ד

Rav Ashi explains that both Moshe Rabbeinu and Dovid Hamelech knew the exact moment of midnight. Then why did Dovid need a harp, since he was up the entire night immersed in prayer and Torah study according to Rav Ashi’s opinion stated earlier (3b) thus not necessitating a harp as an ‘alarm clock’ nor in order to indicate the exact moment of midnight which he already knew on his own?