What does sleep deprivation do to the way in which we perceive various emotional stimuli? A researcher from the Karolinska Institutet in Sweden has written a thesis aiming to answer this question.

Ever felt grumpy after a sleepless night? When we do not manage to satisfy our need for rest, our brains tend to rebel in various ways. Sleep deprivation, studies have shown, can be just as bad as being drunk, as it alters your perception of space and your reaction time. More recent research has even suggested that people who sleep poorly are more likely to shun social contact and be intuitively avoided by others. Since lack of sleep affects the way we see things and interact with others, it comes as no surprise that it can also impact our emotional perceptions, making them likely to be more negative than usual.

In her doctoral thesis, Sandra Tamm, based at the Department of Clinical Neuroscience of the Karolinska Institutet, in Stockholm, Sweden, set out to explore precisely the ways in which sleep loss can alter our emotional perceptions and engagements. Tamm defended her thesis earlier this month.

Sleep loss makes us more negative

In her work, Tamm conducted no fewer than five studies, each of which set out to assess a different aspect of the relationship between sleep deprivation and emotional perception:

• The first study investigated the impact of poor sleep on emotional contagion (a person's ability to mimic and respond to someone else's emotions)
• The second looked at sleep deprivation's effect on a person's ability to empathize with someone else's pain
• The third examined the relationship between sleep restriction and emotional regulation (a person's ability to control their own emotional reactions)
• The fourth looked at sleep restriction and brain network connectivity
• The fifth assessed the impact of seasonal allergy (which is a risk factor for sleep loss) on brain inflammation, aiming to identify a mechanism that could lead to sleep deprivation

All in all, the researcher looked at data concerning 117 participants and used positron-emission tomography (PET) and magnetic resonance imaging (MRI) scans to assess brain activity and brain mechanisms in the context of sleep loss, allergy, and emotional regulation.

The five studies revealed that, indeed, people who experienced sleep loss were more likely to negatively interpret emotional stimuli, a situation called "negativity bias." Moreover, they were also more likely to have bad moods and find it more difficult to regulate their own emotional responses. This is characterized by poor transmission between the information received and processed by the brain and the ensuing emotional behaviors. In her thesis, Tamm summarizes this finding playfully, in haiku form:

After shorter sleep

cognitive top-down control

does not work so well

At the same time, however, the researcher found that sleep deprivation did not significantly impair a person's ability to experience pain empathy, that is, to respond appropriately to someone else's pain. As for the participants with a seasonal allergy—to birch pollen—the researcher reports that they experienced poorer sleep, both during the pollen season and throughout the year, though they managed to get more deep sleep during the pollen season than outside of it.

Tamm also notes that the studies did not reveal any of the brain mechanisms tying sleep loss to negative bias and other changes in emotional behavior. "Regrettably, we were unable to trace the underlying change mechanisms behind sleep deprivation-induced negativity bias by showing differences in the brain's emotional system, as measured by functional MRI," says Tamm. "For people with a pollen allergy, we found signs of inflammation in their blood readings, but not in the brain," she adds. Nevertheless, the researcher argues that her findings contribute to our understanding of sleep deprivation as a top risk factor for poor mental health.

"Ultimately, the results [of this research] can help us understand how chronic sleep problems, sleepiness, and tiredness contribute to psychiatric conditions, such as by increasing the risk of depression," says Tamm.