The Role of Circadian Rhythm in Mood Disorders

Post by Rebecca Hill

How does the circadian rhythm impact mood?

Circadian rhythms are physiological mechanisms that allow humans and many other animals to respond to light and have regular periods of both activity and restful sleep. Circadian rhythms are coordinated by an area in the hypothalamus called the suprachiasmatic nucleus (SCN), which receives direct light input from the retina (Reppert & Weaver, 2001). There is now a growing body of evidence that mood disorders, often diagnosed by abnormal sleep patterns, are associated with disrupted circadian rhythms. These studies have contributed to our understanding of mood disorders and how they can be treated, showing that therapeutic treatments that target circadian mechanisms can often help lessen the symptoms of mood disorders. 

Some of the most common mood disorders include seasonal affective disorder (SAD), major depressive disorder (MDD), and bipolar disorder (BD), with each affecting between 2.8-5% of adults. A core diagnostic symptom of all mood disorders is abnormal sleep/wake patterns. Symptoms for SAD usually start during the change from fall to winter when the daylight hours quickly become shorter (Melrose, 2015). Similarly, manic and depressive episodes of BD are often triggered by seasonal changes (Geoffroy et al., 2014). Patients with BD usually have their sleep/wake patterns disrupted by manic and depressive episodes, which are also in turn triggered by changes to sleep patterns (McCarthy et al., 2022; Malkoff-Schwartz et al., 2000). In MDD, patients cycle through depressive moods throughout the day, with the worst symptoms usually occurring in the early morning (Wirz-Justice, 2022).

The prevalence of depressive mood disorders is increasing, and this could be linked to disrupted sleep driven by the uptick in the amounts of artificial light we are exposed to from phones, computers, and televisions, especially at night (Hidaka, 2012). In addition to this, shift work is common, and forces workers to be awake when their bodies expect to be asleep. This disrupts natural circadian rhythms and may also contribute to the increasing prevalence of mood disorder diagnoses (Boivin et al., 2022).

Neurons signal to adapt to changes in daylight

Midbrain dopamine neurons have been found to be linked to symptoms of depression. Rats exposed to short light days had more dopamine neurons in the hypothalamus that, when damaged, started presenting depression-like behavior (Dulcis et al., 2013). The neurons in the SCN signal at different rates during the summer and winter months, so individuals with SAD may have a SCN that can’t adapt to different seasonal cues (VanderLeest et al., 2007). Manic-like behavior, like that seen in patients with BD, was found in mice with optogenetic stimulation of dopamine neurons, but only at certain times of the day (Sidor et al., 2015). Together, research findings like these indicate that neurons are signaling changes in daylight throughout the seasons, and abnormal signaling could result in the symptoms seen in mood disorders.

Melatonin dysfunction contributes to mood disorders

Melatonin is a hormone released by the pineal gland to indicate darkness and facilitate sleep, meaning more melatonin is released during shorter days. Patients with SAD sometimes have an overproduction of melatonin during the winter and also produce it later in the day than normal, leading to fatigue during the daytime (Lewy et al., 2006; Srinivasan et al., 2006). Melatonin is also produced less, and at inappropriate times of the day by patients with MDD (Pandi-Perumal et al., 2020). Further, individuals with BD are hypersensitive to light at night, which can lead to the suppression of melatonin, and a delay in sleep.

How can we treat these mood disorder symptoms?

Bright-light therapy is the most widely used treatment for SAD. This treatment is typically used in the early morning, since this is the most effective timing window, however, the optimal timing and “dose” of light can vary for each person (Partonen, 1994). Bright-light therapy might work, especially if used in the morning, because it decreases the amount of melatonin being produced at inappropriate times during the day (West et al., 2011), and gives our bodies a strong morning light cue.

Antidepressant medications such as selective serotonin reuptake inhibitors (SSRIs) have also shown promise in helping to reset signaling in the SCN to correct circadian rhythms and decrease depression symptoms (Sprouse et al., 2006). Patients with BD are often treated with lithium, which when used in subjects with shorter circadian periods will lengthen the circadian period, correcting it to the natural 24-hour cycle (Mishra et al., 2021).

What’s next?

For proper mood regulation, the physiological circadian systems must be able to adapt to changes in daylight across the seasons. Individuals with an unstable sleep/wake cycle are more likely to develop mood disorders. Based on recent research, the stabilization of circadian rhythms can often treat the symptoms of mood disorders. Treatments such as bright-light therapy, melatonin, and SSRIs, when personalized to the individual, can greatly improve the outlook for patients with depressive mood disorders. 

References +

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