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Depression and insomnia are like Donald J. Trump and applause — two phenomena that consistently go together for no clear reason. But, we have potential explanations for the recurring appearance of sleep and mood problems, just as we do for the abundance of batty Trump believers.

Some researchers believe that sleep loss compromises people’s ability to regulate emotions, which in turns leaves them more likely to feel depressed. Emotional dysregulation — an inability to be that emotional rock everyone expects — has surfaced across research on sleep loss in different groups of people. Recently, a behavior science team pointed fingers at emotional dysregulation with respect to firefighters’ sleeping issues. They published their explanation this week in the Journal of Clinical Sleep Medicine.

Tragedies don’t follow a 9-to-5 schedule, so firefighters have to battle blazes at odd hours. Given their consistent exposure to death and destruction, the brave men and women may be especially susceptible to the psychological and emotional impact of sleep loss. In the current study, researchers surveyed 880 current and former US firefighters about sleep, nightmares and mental health. Forty percent of participants reported clinically significant depression symptoms, over half reported insomnia symptoms and almost 20 percent said they dealt with nightmares.

The study authors doubt the high incidence of depression is purely a function of firefighters’ occupational hazards. They think poor sleep, on account of stressful shift-work, bears at least some responsibility. For one thing, retired firefighters’ emotional self-assessments resembled those of current ones. For another, insomnia is generally a good predictor of depression — more often than we’d like, today’s sleepless people become tomorrow’s sad people.

Sleep loss doesn't necessarily causes depression in a direct way, study authors explain. Instead, they believe emotional dysregulation is a mediating factor, meaning it acts like a stepping stone between two slippery patches of moss.

In the study, firefighters’ sleep issues separately predicted different mental health issues, including difficulty with emotional regulation (based on a psychological questionnaire) and depression symptoms.

A growing body of research connects loss of sleep — particularly REM sleep — to emotional dysregulation in a number of different contexts. Sleep-deprived people, various studies have shown, become more likely to interpret neutral emotional stimuli (e.g., resting bitch face) as negative, less able to turn the other cheek in response to irritations and less able to apply emotional coping strategies in stressful situations.

People who are moodier, more easily agitated and less equipped to calm themselves down when the going gets tough, researchers surmise, are susceptible to feeling depressed. They’re more likely to see gloom and doom, and thus more likely to sink into a sad state.

At this point, they’ve only built a theory based on previous studies. The sleep-depression link is a hot one in the sleep research world, so, undoubtedly, more research will follow.