Likutei Moharan 54:10/ליקוטי מוהר"ן נ״ד:י׳

110. This is the aspect of waking at midnight. A harp hung over David’s bed, and when midnight arrived it would play by itself (Berakhot 3b). In other words, at midnight the aspect of sacred music drawn from the harp of David is aroused, this being the aspect of extracting the good ruach, etc., as explained above. That is therefore the time to strengthen one’s Divine service: to awaken then to engage in the service of God, to speak at length to the Holy One. For it is mainly then that the above mentioned extracting through the aspect of playing music on an instrument occurs; this being the aspect of the harp of David, which played then, as explained above. Understand well the practical application of these matters.