Monday, 21 March 2011
Ninad Gujar and his colleagues tested 36 participants (half were male; average age 21) on a face processing task, once at 12pm and then again at 5pm. Half the participants were given a 90-minute napping opportunity after the first task, whilst the others just went about their day as usual.
The task involved the participants looking at a computer screen that showed a male face pulling fearful, sad, angry and happy expressions at various intensities. The participants' goal quite simply was to rate each presentation of the face for intensity on a scale from 1 (definitely neutral) to 4 (mostly happy/sad etc).
For participants who stayed awake through the afternoon, their performance at 5pm, compared with at 12pm, demonstrated heightened sensitivity to fearful and angry facial expressions. By contrast, the participants who'd had a nap were less sensitive to fearful expressions at 5pm yet more sensitive to happy expressions. These emotional processing changes were also accompanied by mood differences: the no-nap group reported less positive mood later in the afternoon, compared with earlier, whereas the nap-group reported a decrease in negative mood.
The emotional processing changes observed among the nap-group were related to rapid-eye-movement (REM) sleep. EEG recordings taken while the nappers slept showed that those who obtained REM sleep were more likely to show the desensitisation to negative emotions and sensitisation to positive ones.
An alternative interpretation of the results is that napping affects visual processing, not emotional sensitivity. But the researchers don't think this stands up to much scrutiny, since any basic visual processing effects ought to have been uniform across the different emotions.
So, assuming the emotional sensitivity account is true, why might the non-napping participants have become more sensitive to negative emotions? One possibility, which is backed up by sleep deprivation research, is that the prefrontal cortex becomes fatigued through the day and therefore less able to dampen down emotional reactivity in the sub-cortex. Alternatively, perhaps heightened sensitivity to fear and anger is adaptive - as we fatigue through the day, it makes sense that we should become more vigilant towards these danger-based signals. Either way, a brief nap appears to give us an emotional recharge, altering the way we respond to other people's facial expressions. The implications for working practices are obvious.
'These data add to a growing collection of findings indicating a regulatory role for sleep in the optimal homeostasis of emotional brain function,' the researchers said, 'which if disrupted may have detrimental contributions to clinical symptomotology in affective disorders.'
Gujar, N., McDonald, S., Nishida, M., and Walker, M. (2010). A Role for REM Sleep in Recalibrating the Sensitivity of the Human Brain to Specific Emotions. Cerebral Cortex, 21 (1), 115-123 DOI: 10.1093/cercor/bhq064
Earlier on the Digest: How to nap