The first thought of the day sets the tone for the entire day:[1]
Just like the order in expression of one’s self is thought, speech and action, and ones speech is an outcome of thought, so too it is with regards to the first thought and speech that one has when he wakes up. The first thought and speech one has upon awakening sets the tone for all the thought and speech of that day. Therefore the first thought and speech that one has upon awakening is to be of matters of holiness, in order to draw this holiness into all his thought and speech of that day.
[1] Keser Shem Tov teaching 212; Igara Depirka 205; Derech Pikudecha 18:5; Shulchan Hatahor 3