Is it true that sleeping less than 6 hours can caused early death?

Correlation vs.cause: Both sleeping significantly less than or significantly more than 7 hours a nite for adults is associated with increased mortality but this does not mean that either too little or too much sleep causes this. It is likely that the underlying factors that cause too little or too much sleep are the things that affect mortality, though no one knows for sure. See http://www.nature.com/articles/srep21480