Exhaustion is a vague and forgiving concept. Celebrities say they’re suffering from it when they go to rehab and don’t want to admit to depression or addiction. You can attribute your low mood or your short temper to exhaustion, and it can mean anything from “had a couple of bad nights’ sleep” to “about to have a nervous breakdown.” It also seems like a peculiarly modern affliction. Relentless email, chattering social media, never-ending images of violence and suffering in the news, the lingering effects of the financial crisis, and looming environmental catastrophe: Who’s going to blame you if you confess to having had enough of it all?

Anna Katharina Schaffner’s Exhaustion: A History opens with the resignation of Pope Benedict XVI in 2013. He cited deteriorating physical and mental strength as a major factor in his decision to step down, and Schaffner teasingly holds him up as an emblem of our age, exhausted by the demands placed upon him. Then she points out that the only other pope to resign voluntarily gave reasons very similar to Benedict’s for doing so. That was Celestine V and the year was 1294.

Exhaustion, in popes as in less exalted subjects, is nothing new. But if exhaustion is eternal, our understanding of exhaustion is always changing. Schaffner, who is a lecturer in comparative literature at the University of Kent, shows how each era remakes the condition in its own image, reflecting its medical, technological and cultural developments, as well as its fears. Dangerous precisely because it keeps us from action, exhaustion has for centuries done double duty as a sign of weakness and a badge of honor.

Exhaustion is a floating symptom, and has been attached to numerous different conditions over the past two millennia. Schaffner’s book is more a history of these conditions, many of which are some version of what we call depression, than it is a history of exhaustion itself. The book starts with Galen’s popularization of humoral theory in the second century, and the idea that illnesses were the product of imbalances among the four humors. In Galen’s writing, exhaustion occurs mainly as “lethargy, torpor, weariness, sluggishness, and lack of energy,” all of which he regarded as symptoms of melancholia, produced by an excess of black bile. In this understanding, an exhausted, melancholic mind is the product of a sick body.

In late antiquity, exhaustion was spiritualized rather than medicalized. Under the name of “acedia” (literally “non-caring”), exhaustion signified a lack of faith and an inability to feel and participate in the joy of God’s creation. Monks were particularly at risk from this spiritual sickness, which threatened more than the individual monk suffering from it: A monastic community only functions smoothly when all of its members are pulling their weight. Acedia’s threat to the social order may be part of the reason why religious thinkers viewed it not merely as regrettable but as sinful. Another word for acedia is Sloth, the worst of the Seven Deadly Sins.