How Does Your Circadian Clock Keep Track of the Seasons?

Researchers have identified how circadian rhythms synchronize with the seasons.

Posted Jul 01, 2015

Elena Zajchikova/Shutterstock
Source: Elena Zajchikova/Shutterstock

Until now, the specific neurobiology of how our circadian clocks keep track of the seasons has been a mystery. Recently, researchers led by Toru Takumi at the RIKEN Brain Science Institute in Japan discovered a key mechanism that explains how the brain uses circadian rhythms and length of day to synchronize with the seasons.

The June 2015 study, “GABA-Mediated Repulsive Coupling Between Circadian Clock Neurons in the SCN Encodes Seasonal Time,” was published in the Proceedings of the National Academy of Sciences.

In the new study, the researchers identified a mechanism that encodes the length of a day into the neuronal network of the suprachiasmatic nucleus (SCN). The researchers were able to identify how circadian clock machinery in the brain encodes seasonal changes based on the amount of daylight hours.

The Suprachiasmatic Nucleus (SCN) Is the Master Circadian Clock

The suprachiasmatic nucleus is our master circadian clock. The SCN is also a seasonal clock that measures the length of daylight. The human brain keeps track of the seasons using the same nucleus of neurons that govern circadian rhythms.

National Institute of Health/Public Domain
Source: National Institute of Health/Public Domain

The SCN contains about 20,000 nerve cells and is located in the hypothalamus. The SCN takes the information on the length of day and night from the retina, interprets it, and passes it on to the pineal gland. In response to these signals from the SCN, the pineal gland secretes the hormone melatonin.

The secretion of melatonin peaks at night and ebbs during the day which drives our sleep and wake cycle. Destruction of the SCN results in the complete absence of a predictable sleep and wake cycle. 

What Drives Circadian Rhythms? 

Circadian rhythms are produced by natural factors within the body, but are primarily driven by exposure to light. Disruptions of circadian rhythms are directly linked to sleep disorders. Abnormal circadian rhythms have also been associated with obesity, diabetes, depression, bipolar, and seasonal affective disorder (SAD).

Exposure to sunlight turns the genes that control the circadian clock on and off. Interestingly, the RIKEN researchers found that not all neurons in the SCN march to the same beat. Two regions in the SCN are slightly out of synchrony, and as the length of the day increases, so does the phase gap between these regions.

Courtesy of RIKEN
Source: Courtesy of RIKEN

In a press release, lead auther Jihwan Myung said, "Just like in other animals, our bodies keep track of the seasons. Sudden changes in seasonal day length can cause severe mood disorder in some individuals. Understanding how to adjust our internal seasonal clock could lead to effective ways of helping people whose internal clocks have been disrupted."

The researchers found that the SCN divides circadian clock oscillations into two clusters that correspond with day length. The study reports that the distribution of chloride across the SCN triggers these changes.

More specifically, the researchers found that the neurotransmitter GABA plays an important role in this process. In most cases, GABA inhibits the activity of neurons. However, some SCN neurons are actually excited by GABA.

Myung explains, "GABA becomes excitatory when chloride levels inside neurons are high. We suspected that changes in GABA function across the SCN could represent the repulsive force that pushes these two clusters of neurons out of phase." Coupling estimation by the researchers revealed that the SCN network has couplings that can be either "phase-attractive" (synchronizing) or "phase-repulsive" (desynchronizing). 

Millions Suffer from Seasonal Affective Disorder (SAD) 

Seasonal Affective Disorder is triggered by changes in daylight hours and can lead to depressive symptoms in the winter, and heightened anxiety in the summer. Do you suffer from SAD? 

Treatment for SAD often includes light therapy, also known as “phototherapy." Patients using light therapy typically experience benefits during the first week. Most studies have found that light therapy is most effective if used as a seasonal treatment lasting for several weeks until natural light exposure is possible. 

Neuroscientists are unsure of the exact mechanisms that cause SAD. One theory is that SAD is triggered by a lack of serotonin. Another theory is that SAD may be the result of excessive melatonin produced during winter days. Normally, melatonin levels begin to rise in the late afternoon into evening, remain high throughout the night, and then drop off in the early morning hours before waking up.

Conclusion: Optogenetics Can Reset the Circadian Clock in a Laboratory

Traditionally, neuroscientists believed that the firing rate of SCN neurons was strictly driven by output from the circadian clock's activity. Recently, researchers at Vanderbilt University were able to stimulate or suppress the SCN's neurons in a way that emulates their day and night activity levels. This enabled the researchers to reset the circadian clock in mice.

The February 2015 study published in the journal Nature Neuroscience reported that manipulating circadian clock neuron firing rates using optogenetics can reset the circadian clock. Optogenetics inserts genes that express optically sensitive proteins into target cells that then makes the cells respond to light.

In a press release, Douglas McMahon, lead author of the study said, "We found we can change an animal's sleep/wake rhythms by artificially stimulating the neurons in the master biological clock, which is located in an area of the brain called the suprachiasmatic nucleus (SCN), with a laser and an optical fiber.”

Jeff Jones, who conducted the study with fellow doctoral student Michael Tackenberg added, "This puts clock neurons under our control for the first time." Although this study was done in mice, the researchers are optimistic that sometday optogenetics could be used as therapy for circadian clock disruptions in humans. 

If you'd like to read more on this topic check out my Psychology Today posts:

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