Question : What is a mantra? Is it merely a combination of words?
Answer : Let's understand it with this example : Suppose a yajna (fire sacrifice) is being performed simultaneously at 300 different places exactly at the same time. They first do oblations called aahuti. They take the name of the deity to whom it is supposed to go. Indrayedam na mama, Indraya swaha. May this be given to Indra. Now the question is where will Indra go first and how at the same time all the different locations? The answers is, the words are the sound energy and when these specific sounds are made it takes the form of Indra as you have imagined and hence comes in the form of that deity and accepts the oblation.
Hence mantras are not only mere combination of words or descriptions of God but they are God themselves. God has got this form also. He is in the form of sound energy. All these bija aksaras are not empty mumbo jumbo. They are all potent power of the Divinity.
Hence, when you keep chanting it day and night, that sound effect makes you get up from this so called waking state.
Please check these: Excerpts from a discourse by our Sri Sri Muralidhara Swamiji