I believe this song is autobigraphical. The fireflies are symbolism for illuminating and inspiring thoughts that fill his mind, lighting it up even as he remains awake whilst desirous of falling asleep. The singer is at first torn between wanting to remain in the flow of inspiraton and revelation of His Creator and the wonders of the created world, and wanting to fall into a sound dreamless sleep. This altered last repitition of the chorus is for me the key to all that goes before. The Singer/songwriter revels in his triumph over sleeplessness because he is using his insomnia creatively to tap into the well of inspiration that is rising up with in him as he spends the sleepless nights in the presence of the Creator, through which he himself becomes creative. He's living his dreams even as he suffers from sleepless nights brought on by insomnia. And the song is a testimony to that triumph.