The 10 Things I Hate About You star sadly passed away on January 22, 2008, at the age of 28 after taking a variety of pills.
Following his passing, Heath's father Kim recalled that his daughter Kate had told her brother the night before his death: "You can’t mix drugs that you don’t know anything about."
However, Heath is said to have told his sister: "I’ll be fine," but just one day later he was found dead in his apartment by his massage therapist Diana Wolozin.
In the autopsy the New York coroner found that the father-of-one had died of "acute intoxication" after ingesting a cocktail of prescription medication. His death was ruled an accident.
Anti-anxiety drugs Valium and Xanax, the painkiller OxyContin, cough suppressant hydrocodone and sleep aids Restoril and Unisom were all found in his system among others.
In 2017, forensic pathologist Dr Jason Payne-James said that it was the addition of OxyContin and hydrocodone that proved fatal for Heath.
"It just put his whole system to sleep I guess," his dad Kim told news.com.au of the coroner's findings.
"It was a one-off thing. That’s what killed us, because he was warned by his sister the night before: 'You shouldn’t mix what you’re taking for pneumonia with your Ambien'.
"He was a young guy that traveled all the time for work. Even as a two-year-old, he hardly ever slept. He was trying to work and travel and do everything in a short space of time."
He continued: "Heath mixed a couple of drugs together with sleeping tablets and he’s gone forever."
Heath had spent the months before his death immersing himself into the role of The Joker for Christopher Nolan's Dark Night.
He had recently moved into a Manhattan rental apartment after splitting from girlfriend and mother of his daughter, Michelle Williams.
His friend - and sometimes housemate - Gerry Grennell claimed that Heath was desperate to sleep and had been suffering from "walking pneumonia".
He told People magazine: "I would hear him wandering around the apartment and I’d get up and say, ‘Come on, man, get back to bed, you have to work tomorrow'. He said, ‘I can’t sleep, man'.
"He missed his girl, he missed his family, he missed his little girl - he desperately wanted to see her and hold her and play with her. He was desperately unhappy, desperately sad."
Gerry continued: "He said, ‘I got to stop, it’s not helping, I’m not well, it’s making me feel more upset'.
"It wasn’t helping with the relationship issues, it wasn’t helping with missing his kid, it wasn’t helping his sleeping - and he knew that.
At the time of his death, The Dark Knight was in post-production and Heath’s performance as the Joker went on to earn him universal acclaim and popularity from fans and critics alike.
He also received numerous posthumous awards for role, including the Academy Award for Best Supporting Actor.