Sleep-deprived people are known to have stronger reactions to negative experiences, but some patients with depression seem to perk up with lack of sleep. To resolve this paradox, Matthew Walker at the University of California, Berkeley, and his team used functional magnetic resonance imaging to scan the brains of 14 people who hadn't slept for about 36 hours while presenting them with emotionally neutral and pleasant-looking images.

The volunteers rated a greater proportion of the images as 'pleasant' than did people who had maintained a normal sleep routine. The sleep-deprived individuals also showed increased activation in brain regions that mediate reward-driven behaviour, as well as greater connectivity between certain visual and emotional processing centres.

The results suggest that sleep deprivation can enhance reactivity to both positive and negative stimuli.

J. Neurosci. 31, 4466–4474 (2011)