WASHINGTON (AP) — Don’t lose sleep over headlines linking melatonin to heart failure. That’s the message after some scary-sounding reports about a preliminary study involving the sleep-related supplement. It raised questions about the safety of long term use of melatonin for insomnia. Doctors have long known that too little or interrupted sleep raises the risk of heart disease. But heart experts say this kind of so-called observational study can’t prove that melatonin use plays any role —