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Imagine your body as a finely tuned orchestra, where the circadian rhythms are the conductors, orchestrating our biological processes throughout the day and night. These rhythms help us synchronise with the 24-hour cycle of each day. But what happens when the conductor goes offbeat? In the realm of mental health, particularly with bipolar disorder, the consequences can be profound. This blog delves into a study that asks a crucial question about how our internal clocks— circadian rhythms—impact our brain’s reward system, and how mood symptoms might impact that.

Circadian Rhythm and Sensitivity to Reward

Circadian rhythms, the natural cycles that regulate our sleep-wake patterns and numerous other bodily functions, play a crucial role in our overall well-being. These rhythms influence everything from hormone release to body temperature and cognitive functions (e.g., decisions we make and actions we take). Further, our circadian rhythms could even impact when we feel most motivated and eager to seek out rewards throughout the day. Reward sensitivity is a fundamental aspect of human behavior, influencing how we respond to positive stimuli and pursue goals. Also, theories suggest that we have evolved to be good at managing our energy reserves. Therefore, the synchronisation of circadian rhythms with our reward system ensures that we are primed to seek and savour rewards at optimal times, enhancing survival and efficient use of energy reserves.

Integrating Circadian Rhythm, Reward Sensitivity, and Bipolar Disorder

Caught in a delicate balance, our circadian rhythms and the pursuit of pleasure shape our every day, but for those with bipolar disorder, this balance is often disrupted. Characterised by profound mood swings from the highs of (hypo)mania to the lows of depression, bipolar disorder usually presents irregularities in both circadian rhythms and reward processing. Individuals with this condition often struggle with the timing of their reward-seeking behaviors—intensifying these pursuits during the day when feeling depressed, and struggling to wind down at night, which can manifest as hypomanic symptoms. The intertwining of reward hypersensitivity with disrupted circadian rhythms could be exacerbating these challenges.

Yet, there’s still much we don’t understand about the exact pattern of how reward sensitivity changes during the day in the context of mood symptoms with neuroimaging evidence. Here, we hypothesised that:

• Reward sensitivity would be greater in the afternoon than mornings and evenings.
• Subsyndromal mood symptoms (depression and hypomania) could be linked to less variation in how rewards feel throughout the day.
  

The Current Study

The study used data from the IMAGEN project, involving over 2000 people aged 13 to 14. This vast dataset involved using fMRI (i.e., a technique for measuring brain activation in response to rewards) to observe how different brain regions involved in reward processing responded under The Monetary Incentive Delay (MID) task, and self-rated mood symptoms.

What is the MID task?

The MID task is a neuroscience research tool used to evaluate how individuals anticipate and respond to potential rewards, the gain or loss of money.  During the MID task, participants will see cues and have to quickly press a button. These cues show if they can win money, prevent losing money, or neither. We used this task to study brain activity that shows how people expect rewards and react to them.

We particularly focused on whether anticipation of reward in the brain fluctuated with time of day and different mood symptoms.

What have we found?

The study noted some brain regions known for their roles in the reward system showed a reward sensitivity modulation during the day, like the ventral tegmental area and left putamen. However, the change in reward sensitivity did not align with our expectation of peaking in the afternoons and declining in the mornings and evenings.

For the mood symptoms, contrary to expectations, the severity of mood symptoms (depressive or (hypo)manic) did not significantly alter the reward sensitivity in the brain. This finding challenges some existing theories about how mood symptoms directly affect brain function in bipolar disorder.

While the patterns of reward sensitivity didn’t match our initial expectations, an intriguing discovery emerged regarding biological sex differences. The study uncovered significant variations in how reward sensitivity is modulated throughout the day between males and females, hinting that circadian influences might impact biological male and female brains differently. This opens up fascinating avenues for exploring how gender-specific biological rhythms could shape our mental health responses.

Insignificant Results: Equally Enlightening as Significant Findings

However, the study’s findings also underscore the complexity of the mechanisms between time of day, mood symptoms, and reward sensitivity. The lack of a significant direct correlation between mood symptoms and reward processing suggests that other factors, perhaps genetic or environmental, may also play critical roles.

This study not only adds a layer of understanding to how circadian rhythms might influence reward sensitivity but also opens new avenues for managing bipolar disorder. If circadian rhythms indeed affect how individuals with bipolar disorder respond to rewards, treatments that stabilise these internal clocks could potentially mitigate some of the disorder’s impacts.

Challenges in our Research: Methodological Considerations

Several factors contribute to these findings, including the different patterns of circadian rhythm among individuals and challenges in measurement accuracy. For instance, studies often rely on community-based samples that might not display pronounced symptoms, diluting potential effects. Furthermore, the timing of assessments can misalign with participants’ biological clocks, skewing results. These complexities underscore the need for refined methodologies and larger and more diverse samples to capture subtle but clinically meaningful effects. Understanding these limitations is key to advancing our knowledge and improving interventions for mood disorders.

Conclusion

This exploration into the intricate ties between our internal clocks, reward systems, and mood disorders like bipolar disorder illuminates promising pathways for therapeutic advancements. By understanding how body clocks influence reward sensitivity, particularly in the context of bipolar disorder, we can better tailor treatments to realign these rhythms, potentially easing the severe mood swings associated with the disorder. This study underscores the importance of considering our biological clocks in developing effective mental health interventions, offering hope for more personalised care strategies.

References:

1. Dijk, D. J., & Lockley, S. W. (2002). Invited Review: Integration of human sleep-wake regulation and circadian rhythmicity. Journal of applied physiology, 92(2), 852-862. https://doi.org/10.1152/japplphysiol.00924.2001
2. Kim, S. H., Yoon, H., Kim, H., & Hamann, S. (2015). Individual differences in sensitivity to reward and punishment and neural activity during reward and avoidance learning. Social cognitive and affective neuroscience, 10(9), 1219-1227. https://doi.org/10.1093/scan/nsv007
3. Murray, G., Nicholas, C. L., Kleiman, J., Dwyer, R., Carrington, M. J., Allen, N. B., & Trinder, J. (2009). Nature’s clocks and human mood: The circadian system modulates reward motivation. Emotion, 9(5), 705. https://doi.org/10.1037/a0017080
4. Plante, D. T., & Winkelman, J. W. (2008). Sleep disturbance in bipolar disorder: therapeutic implications. American Journal of Psychiatry, 165(7), 830-843. https://doi.org/10.1176/appi.ajp.2008.08010077
5. Alloy, L. B., Boland, E. M., Ng, T. H., Whitehouse, W. G., & Abramson, L. Y. (2015). Low social rhythm regularity predicts first onset of bipolar spectrum disorders among at-risk individuals with reward hypersensitivity. Journal of abnormal psychology, 124(4), 944.  https://doi.org/10.1037/abn0000107
6. IMAGEN Project, see https://imagen-project.org/

Credits:

1. Cover picture from iStock Photos: https://www.istockphoto.com/photo/the-circadian-rhythms-are-controlled-by-circadian-clocks-or-biological-clock-gm1361759855-434029266
2. Icons for the first picture in the main text from Iconfinder: https://www.iconfinder.com/