Here is a fascinating case of someone without vital signs for 18 hours (that's almost one full day) yet he was revived. It is believed the man survived a heart attack because hypothermia caused his body temperature to plunge. This is not the only case of people being pronounced dead and then many hours later coming 'back to life'. This particular case may be explained by the cold temperatures (or maybe it was just coincidence) but there are other cases where people are known to have awakened suddenly after being dead for hours and there were no temperature influence. Give this a read after: Man Who was Declared Dead by 3 Doctors Wakes Up in Morgue Just Hours Before Autopsy via unexplained-mysteries: The 53-year-old was resuscitated despite lying lifeless outside with no heartbeat for the better part of a day. The man, whose name has not been disclosed, was only saved because the near-freezing conditions outside had brought on hypothermia, causing his body's core temperature to plummet.