Lyric discussion by prettybird 

There's a theme of sleeping, dreaming, and waking throughout this one:

"When I awoke, you were standing there." "There's no falling back to sleep once you've wakened from the dream." "Now, I'm rested and I'm ready to begin."

I don't think specifics (such as addiction or adultery, as others have suggested) are relevant unless the songwriter actually mentions or implies them in the song. The writer (which is Scott, in this case, as the Spotify commentary would tell us) includes the details that matter.

I think this song is about maturing, and transitioning from the bad habits of youth and becoming an adult. Once you realize the truths that dawn on you as you grow older, you can't go back to thinking the way you used to. ("There's no falling back to sleep once you've wakened from the dream.") At the same time, the last line of each verse emphasizes that someone has been loyal to him throughout his emotional relapses and shortcomings (the variations of "I awoke and you were standing there"). So I think it is also about being grateful for someone who has loved you at your worst as well as your best, and the importance of loving someone that faithfully.

My favorite line in this song is "the light broke in and brought me to my feet". Seth's harmony on that line is absolutely heartrending. I kind of wish it were "YOUR light broke in and brought me to my feet" :)

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