Being in a bad mood can be good for your health (a recent study has shown), but out of respect for others, wouldn’t it be nice to know what puts us in a bad mood? All of us have had the opportunity to see what lack of sleep does to our mood the next day. But we know less about the adverse effects of being unexpectedly awoken during the night. A recent study published in the journal Sleep answers just this question: why does interrupted sleep put us in a bad mood?

Earlier studies have already highlighted that sleep interruptions, even short ones, can disturb sleep quality, leading to a sleepless night, with the usual effects on mood (not to mention fatigue and trouble focusing and paying attention). A single night of interrupted sleep may even be enough to alter our mood (Sleep Medicine - "http://www.journals.elsevier.com/sleep-medicine", published on 26/09/2014). This study confirms these results.

In order to determine the effects of multiple sleep interruptions on mood, P. Finan and his colleagues at John Hopkins University randomly subjected 62 men and women to 3 different sleep scenarios in a hospital environment. Participants in Group 1 were awoken eight times during the night (for three nights), those in Group 2 went to bed late, and those in Group 3 were left to sleep normally and for a longer period than those in the two other groups. The sleep architecture was assessed using polysomnography, which involves recording several physiological variables (respiratory rate, heart rate, EEG), mood was also self-evaluated with an estimate of positive emotions (such as joy) and negative emotions (such as anger).

As one might guess, from the first night, participants in Groups 1 and 2 experienced more negative emotions when compared to group 3. The second night was of particular interest as only the subjects in Group 1 (interrupted sleep) saw their positive emotions decrease by 31%, while they decreased by only 12% for subjects in Group 2 (later bedtime). In other words, the people in Group 1 were none too happy. Sleep patterns can play a role in the onset of a bad mood. Indeed, fewer slow waves, characteristic of deep sleep were observed in the participants of Group 1.

More than lack of sleep, it's the quality of sleep that appears to affect mood; unexpected awakenings reduce deep sleep, a necessary stage for rest and recovery. The inadequacy of this “recovery” sleep explains the decrease in positive mood.

The aim of the study by P. Finan and his colleagues was to test an experimental model of the effects of disrupted sleep continuity on sleep architecture and positive mood in order to better understand the mechanisms involved in the relationship between insomnia and depression. The results of their research may also be useful in understanding and preventing depression.