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.
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.