While one of the key misconceptions about depression is that it’s simply prolonged sadness — such a condition is more commonly known as mood disorder — the mental illness may in fact be predicted by a person’s declining mood, a new study finds.

It’s thought that the slide from depressed mood to clinical depression is marked by an increasingly steep curve. At a certain point, the slightest push can lead to an enormous decline, one that has come to be known as the “tipping point.” From fashion trends to global climate changes, tipping points represent a small cluster reaching critical mass before diffusing to the surrounding population at large. Among depression sufferers, this tipping point, if harnessed completely, could hold the potential to report when intense sadness has actually turned into something more.

“The holy grail of depression epidemiology is that we want to intervene early to prevent people from having depressive episodes,” Harvard University social scientist, Stephen Gilman, who was not involved in the study, told Science. “Where this work is headed is making an advance in that direction, toward early detection and therefore early intervention.”

The latest study recruited 600 people, both depressed and not, to wear a watch for five to six days that beeped 10 times per day. When the watch beeped, subjects were asked to record their present mood according to four measures. Were they cheerful? Content? Sad? Anxious? After six to eight weeks, the researchers had each participant fill out a questionnaire detailing their level of clinical depression. They found 13 percent of subjects had become further entrenched in their depression, a level the researchers said fit with their expectations.

Tipping points typically follow, what researchers call, a critical slowing down. This refers to how long it takes for a system to return to equilibrium. In people with mood disorders, being yelled at by one’s boss and coping with it the next day may eventually slow into never coping with it, until each negative emotion sticks with the individual and gets internalized as a trait of personhood. It’s in this positive feedback loop of sadness, stress, fatigue, and self-doubt that the tipping point for major depressive disorder typically lurks.

“In any system, if you push the system a little bit out of equilibrium, then the closer it is to the tipping point, the longer it takes to return to equilibrium after that perturbation,” explained Ingrid van de Leemput, study leader and ecologist at Wageningen University and Research Center in the Netherlands, to Science. Van de Leemput and her team found that the longer someone took to recover from feelings of sadness and anxiety, the more likely they were to be diagnosed with depression at the end of the study. She and her colleagues hope one day to develop a smartphone app capable of checking mood and assessing the user’s risks.

As a neurological and chemical imbalance, major depressive disorder affects roughly 10 percent of the American population, according to the Centers for Disease Control and Prevention (CDC). The influx of anti-depressant medication over the last two decades serves as a testament to the prevailing trend to medicate illnesses after the fact, rather than to seek ways of preventing them from ever arising. As for a potential smartphone app, researchers are hopeful the path could alert people to conditions they’d otherwise chalk up to simple follies of emotion.

“I think this could open up new avenues of research in many ways,” Gilman explained. “Really what we want to know is where on the distribution of sadness and mood is the dividing line between a serious depressive episode and non-depression. And are there factors that can push people further from or towards that dividing line?”

Source: van de Leemput I, Wichers M, Cramer A. Critical slowing down as early warning for the onset and termination of depression. PNAS. 2013.