Sunrise Sunset Lyrics
I like the yourstruly session better
I think it's about depression and suicide. He doesn't want to stress out the person he's with with his problems i.e. Your heart slows and I'll cry until you wake up. He loves her and is really dependent on her but he doesn't want to be. He wants to feel better instead, and wants to have his existential depression without upsetting his lover. I feel like this is something any artsy depressive type can relate to.
I think that the author/singer is keeping watch over a suicidal lover. They're arguing and he tells her he's leaving to be alone. I don't think she threatens it outright, but maybe she had attempted in the past? He remembers this before leaving and decides to either stay until the sun rises again. This is either literal - he's staying through the night because he could call her friends/family in the morning maybe, or figurative - he's staying until she's not so depressed. The whole "Stop. Your heartbeat will fall. And I will cry until you wake up." is him telling her not to do it, because he won't stop crying if she was successful.