We all have first-hand experience of poor or restricted sleep and the negative impact it has on how we feel and function the next day. While we are familiar with the subjective impact of sleep loss on our mood and often increased levels of irritability, it is only now that researchers are trying to uncover how the brain processes emotional information after lost sleep.

For example, in 2007 researchers from the University of California, Berkeley used functional magnetic resonance imaging (fMRI) to study the brains of sleep-deprived participants whilst they viewed negative arousing images (e.g. car crash scene). Their results showed that emotional regions of the brain (namely the amydalae) became hyperactive relative to a group of participants who were well-rested.

In a recent study, published in PLoS One, Motomura and colleagues extend this work by investigating sleep restriction – a type of sleep loss that we might all commonly encounter on a weekly/monthly basis. The research group from Japan recruited healthy adults and asked them to spend just four hours in bed per night for five days, and then another five days on a regular schedule of 8 hours time in bed per night.

At the end of each of the five day ‘blocks’, participants slept in a sleep lab and, the next day, were asked to passively view faces (happy and fear facial expressions) while having their brain scanned using functional magnetic resonance imaging. Participants also completed measures of sleepiness, anxiety and mood during the two different five-day blocks (sleep restricted versus normal sleep-time). For most of the nights participants implemented the new sleep time at home and were monitored with an actiwatch device which measures movement (the lack of which is a proxy for sleep).

When implementing the sleep restriction protocol, participants slept for approximately 4.5hrs, compared with approximately 8 hours when implementing the control condition. During these periods of sleep restriction, participants also reported greater feelings of sleepiness and anxiety.

The main findings were that sleeping just 4.5 hrs over the 5 days, led to an increase in brain activity in the left amygdala when viewing fearful face expressions. There was no difference however, in brain activity for viewing of happy face expressions. The amygdala serves an important role in monitoring and detecting threat in the environment; and has been found to be hyper-responsive in a number of psychiatric disorders.

The authors also looked at how emotional brain areas interacted when viewing faces, finding reduced connectivity between two key regions (amygdala and ventral anterior cingulate cortex) when participants were sleep-restricted. Furthermore, the size of this decrease in brain connectivity was related to increased ratings of anxiety (relative to when participants slept in the 8hour condition).

Although the study has some limitation relating to lack of control over sleep in the home environment, the findings shed new light on how relatively normal levels of sleep loss – that many of us impose upon ourselves each week – can alter how the brain processes emotional information.

The authors also note the strong relationship between sleep disturbance and mental health, and suggest that chronic sleep loss may, over time, increase the vulnerability to depressive symptoms and compromised mental health.

Original paper:

Motomura, Y., Kitamura, S., Oba, K., Terasawa, Y., Enomoto, M., et al. (2013). Sleep debt elicits negative emotional reaction through diminished amygdala-anterior cingulate functional connectivity. PLoS ONE, 8(2), doi:10.1371/journal.pone.0056578