It is estimated that 322 million people worldwide suffer from major depression every year. Depression is a serious mental health condition that can have debilitating effects on the individual and is a leading cause of disability. Understanding how this condition develops and identifying risk factors are key to developing strategies that work to prevent depression.
Over the last 10 years, increasing evidence indicates that healthy people with a late chronotype may be more at risk of developing depression. Chronotype is a trait that is present in the healthy population and refers to individual differences in diurnal preference; for example, late chronotypes (or ‘night owls’) prefer to go to bed late, wake up late and perform demanding tasks later in the day, whereas early chronotypes (or ‘early birds’) prefer to rise early and would typically schedule meetings, exercise sessions etc. earlier in the day (i.e. synchronised to their circadian rhythm). The majority of people fall into an ‘intermediate’ chronotype category (in-between early and late chronotypes).
Although evidence suggests a relationship between late chronotype and depression, the mechanisms underlying this association are still poorly understood. The following paper entitled ‘Altered resting-state connectivity within default mode network associated with late chronotype’ investigated this further by looking at differences in brain connectivity of late chronotype individuals at rest using functional Magnetic Resonance Imaging (fMRI) (termed resting-state fMRI).
The brain is never truly at rest: there are a number of brain regions known to be active even when we are not focused on anything in particular. Brain regions that activate together at rest are functionally connected and are organised into ‘resting-state networks’. A key resting-state network is the Default Mode Network (DMN). This network is activated when someone is internally focused; for example, during daydreaming, planning for the future and retrieving memories. A number of previous studies have shown that this network is disrupted in people with depression and in a number of other at-risk groups (e.g. people with a genetic risk for depression) and this is thought to contribute towards negative thinking patterns often associated with depression. Therefore, the aim of this study was to investigate whether late chronotype individuals have altered resting-state connectivity within the DMN, similar to depressed individuals, which may help us to explain why they are at increased risk for developing the disorder.
In this study, forty-six healthy participants underwent resting-state fMRI and were asked to remain still in the MRI scanner, with their eyes open but not to think about anything in particular i.e. let the mind wander. We measured their chronotype using a well-validated questionnaire. We also asked them for information about their age, gender, sleep quality and mood in order to exclude these effects on the results using statistics.
We found that people with a later chronotype had reduced connectivity in a specific brain region of the DMN; the precuneus. In previous studies, the precuneus has been shown to be important for thinking about the self and, in particular, reduced connectivity within the precuneus may indicate self-critical thoughts. Some of our previous work has shown that compared to early chronotypes, late chronotypes take longer to recognise likeable words and remember more dislikeable words when these words are used to describe themselves. This suggests late chronotypes have negative perceptions of themselves. Therefore, our findings suggest that alterations within this key brain network at rest represents a mechanism underlying self-critical thoughts in late chronotype individuals.
Importantly, this finding may help us to explain why these people are more vulnerable to depression (although this does not mean that they will develop depression). This may aid us in developing strategies to prevent the onset of depression and promote positive mental health and well-being in these people; such as cognitive bias modification and adjusting our daily schedules to better match our internal body clocks.
These findings are described in the article entitled Altered resting-state connectivity within default mode network associated with late chronotype, recently published in the Journal of Psychiatric Research. This work was conducted by Charlotte Mary Horne and Ray Norbury from the University of Roehampton.