Our bad mood may be symptomatic of more than just a hard day, according to research published in Brain, Behavior, and Immunity. It may signal poor health – specifically poor health related to inflammation. 

Researchers from Pennsylvania State University tracked the mood of 220 volunteers to study associations between negative emotions and levels of inflammation. But instead of relying on answers to questions asked retrospectively (which can be extremely vulnerable to memory bias and personality factors) the researchers tested a phenomenon called ecological momentary assessment (EMA).

Ecological momentary assessment is effectively the real-time monitoring of emotions. During the study, participants' phones would light up five times a day, prompting each to rate on a scale of one to seven how much they felt certain positive affect emotions (happy, alert, enthusiastic, excited, cheerful, relaxed, content, peaceful, calm, and satisfied) and negative affect emotions (irritable, sad, tense, bored, stressed, depressed, nervous, sluggish, upset, and disappointed). After two weeks, volunteers were also asked to recall the positive and negative emotions they felt during that 14-day window, and had their blood drawn to be tested for concentrations of C-reactive protein (CRP) and inflammatory cytokines. 

As they hypothesized, the researchers found that there was a correlation between negative mood (reported at the time) and inflammation but interestingly, only when they analyzed the results of the second week alone – the week leading up to the blood draw. In contrast, taking the results of the entire two-week period did not reveal any link between recalled or momentary reporting and inflammation biomarkers.

This suggests there is at least some link between mood and health, but to what extent and how reliable that association is, is still uncertain. As the researchers themselves point out, the study needs replication and the hypothesis needs further investigation.

While the researchers made sure to control for certain factors like age, sex, BMI, and smoking, it is also worth noting that 90 percent of the participants here had at least one chronic health condition and 22 percent had been flagged for moderate or more severe depression – both of which may have influenced the results at least somewhat. However, it does seem to add to a growing body of research linking our mental health to our physical health. That is, if you've been feeling unusually down lately, it might be time to see a doctor.