Don't think there has ever been a case of a human dying from sleep deprivation, there was a study done in the 1980's with rats where the researchers kept them awake all the time and it took 32 days before all the rats were dead however it wasn't actually proven how the rats actually died as there were three general lines of thoughts of what caused their death, could have been hypothermia, stress or immune system failure however all three would have been caused by the severe sleep deprivation.