Chapter 5 - Number Theory and the Real Number System - 5.2 The Integers; Order of Operations - Exercise Set 5.2 - Page 270: 126

$4$ years

A person who gets less than 6 to 8 hours of sleep per night will shrink his/her lifespan by 1 year. The number that represents this is $-1$. A person who feels frequently stressed shrinks his/her lifespan by 6 years. The number that represents this is $-6$. Thus, the difference between the lifespan of the two is: $=-1 - (-6) = -1+5=4$ years