Med thumb basketcase photo

For the most part, sleep deprivation turns people into blunted, powered-down versions of their well-rested selves. Reaction times lag. Opportunities for snarky quips go un-seized. Otherwise fluid mental processes, like switching between conversation topics, become taxing.

Though skipping sleep appears to impair most cognitive functions, it certainly doesn’t turn down emotions — quite the opposite, in fact. After pulling an all-nighter, even the calmest, meekest of souls can fly off the handle like a Real Housewife who’s been excluded from a girls’ weekend. From observing both crankiness-in-action and brain activity changes in neuroimaging studies, we know that sleep deprivation has different effects on how we think and feel.

But why, exactly, does exhaustion breed basketcases?

A recent study from behavioral scientists at Florida’s Nova Southeastern University suggests the answer hinges on differences in how well-rested people and poorly rested people pay attention to the world around them. According to the research, published online in The Journal of Sleep Research in April, sleep-deprived people exhibit a negative attentional bias, meaning they unconsciously focus on negative environmental stimuli, such as sad faces, foreboding sounds or threatening language.

Their Debbie Downer tendencies make sense from an evolutionary perspective, explained the study’s lead researcher, Jaime Tartar. Tired people have less mental energy than their bushy-tailed counterparts; therefore they use their limited resources for tasks that are critical to survival. From an evolutionary perspective, pinpointing potential danger matters more than solving brain teasers.

The study:

One hundred fifty two college students filled out questionnaires about their sleeping habits, which included assessments of how much and how well they rested, in addition to which hours they slept. Then, participants completed tasks designed to gauge their cognitive and emotional states. The cognitive task measured their ability to pay attention for a sustained period of time. In the emotion-gauging task, participants watched a variety of faces flash across a screen and, as instructed, pressed a button whenever they perceived a face as negative.  

Essentially, researchers were looking to see if people with poor sleeping habits would generally perform worse on the attention test, indicating cognitive impairment, but still exhibit heightened awareness in the emotional task.

In the end, students’ self-ratings of their overall sleep quality was the strongest indicator of their performance on the tasks. Students who said they slept worse were more likely to rate a face as negative. This made them more accurate when picking out truly negative faces, but also more likely to over-interpret neutral faces as negative.

Basically, they were primed to see “bad,” whereas better-rested students weren’t seeking out signs of negativity. The bad sleepers also performed worse on the attention test, suggesting that they were, as a fair amount of research has suggested, not thinking as sharply as they might on more sleep.

Underslept students also reported experiencing more symptoms of anxiety and depression. Other studies have shown a negative attentional bias in people with diagnosed mood disorders. So while the relationship between poor sleep, mood disorders and being a negative nancy isn’t totally clear, it’s worth looking into further.

It’s possible that underlying depression predisposes people to sleep poorly. It’s also possible they feel depressed because they’re so spent.

Negative attentional bias is a type of cognitive bias, which collectively refers to irrational judgement flaws that we all acquire and use to make sense of the world around us. As Tartar pointed out, even if we can’t control biases easily, it’s important to understand when and how they influence behavior

“Findings from this study and similar ones suggest that those most at risk would be individuals who routinely need to rely on good emotion processing while making critical and quick decisions," said Tartar. "For example, E.R. physicians and nurses, paramedics, other rescue workers and soldiers. The extent to which sleep hygiene can affect their performance should be considered when setting long hours or shift work schedules.”