Have you seen him lately?
He looks like death warmed over.
https://www.google.com/search?num=100&safe=off&site=&source=hp&q=david+letterman+current+photo&oq=david+letterman+recent+photo&gs_l=psy-ab.1.1.0j0i22i30k1l3.86285.92163.0.95979.30.28.0.0.0.0.278.3731.0j21j2.23.0....0...1.1.64.psy-ab..7.23.3725.0..0i131k1.2oEje6qc6ZI
The shock treatments must have helped.