underload syndrome
n. Ill health or depression caused by a lack of challenges or stimulation at work.
Prof. Ad Vingerhoets of Tilburg University in the Netherlands coined the term leisure sickness to describe the nausea, fatigue, headaches and recurrent infections that strike some people when they stop working for more than a few days.

Vingerhoets has estimated three per cent of the population suffers from this condition, which is also called underload syndrome.
—Tom Keenan, “A regular vacation works wonders,” The Calgary Herald (Alberta), March 09, 2006
People who lack stimulation in their working lives are likely to be depressed and suffer from headaches, fatigue and recurrent infections, all symptoms of "underload syndrome".
—Hermione Eyre, “A nerve-racking, stressful, exhausting life? Yes, please!,” The Independent, May 04, 2003
2003 (earliest)
And it's not just our leisure time that is putting our health at risk. A lack of stimulation at work can have similar negative effects — known to psychologists as "underload syndrome". Studies at the University of Northumbria found that bored people have more days off sick than any other group.
—Lucy Elkins, “Bored sick,” Sunday Times of London, February 02, 2003