Rep. Sean Casten Reveals Teen Daughter Died After Heart Suddenly Stopped

Rep. Sean Casten is revealing how his daughter died unexpectedly earlier this summer -- saying her heart simply gave out ... without any explanation whatsoever.

The U.S. congressman from Illinois broke the sad news Friday -- elaborating on his earth-shattering post from June ... when he told the world his 17-year-old, Gwen, had passed away suddenly. At the time, he didn't share any details ... but now, he's shedding some light.

Casten says Gwen died of a cardiac arrhythmia -- adding, "[S]he was fine, and then her heart stopped. We don't know what caused the arrhythmia, and likely never will."

The Representative says his daughter was a healthy teen who got regular checkups, exercised, ate well, didn't suffer from any behavioral health issues and had strong relationships with her friends and family. He notes ... she was also fully vaccinated.

Casten says Gwen came home one night after a night out with friends, and didn't wake up the next morning -- something doctors haven't been able to make sense of ... and ditto for the family. He says they're completely heartbroken and devastated -- exacerbated by such a freak thing with no real rational reason behind it. This is rare, he says, but it happens.

He ends his thoughts with a poignant remark ... "None of us know when our last heartbeat will come. The best we can hope for is when our loved ones do pass, we will have no regrets about the time we were lucky enough to share."

Casten finishes with this -- "So hug the folks you love a little harder today. Be present in their lives. And spread your love a little farther. Because in the end, it's all that matters."