Cognitive and Social Neuroscience at the University of Chicago and a leading authority on the topic. There is mounting evidence that loneliness significantly increases the chances of diabetes, sleep disorders, and other potentially life-threatening problems. Research has also shown a greater risk of high blood pressure among lonely people, as well as higher levels of the stress hormone cortisol, weakened immune systems, and Alzheimer's disease.
http://www.aarp.org/personal-growth/transitions/info-09-2010/all_the_lonely_people.html