By: Caitlin Millett, 2nd year PhD candidate in the Neuroscience Program
While the majority of us look forward to seasonal festivities, millions can also expect feelings of depression, fatigue, irritability and poor sleep.
This form of mental illness, commonly known as the winter blues, is Seasonal Affective Disorder (SAD). SAD is disproportionately represented in populations furthest from the equator. It is estimated that 1-2% of North Americans have a mood disorder with a seasonal pattern, with 10% of New Englanders versus 2% of Floridians affected. Symptoms of SAD include feelings of hopelessness, low concentration, sluggishness, social withdrawal, unhappiness and irritability.
Decades of research has uncovered the culprit behind this debilitating illness: lack of sunlight and disruption of circadian rhythms.
Although most people are able to adequately adapt to the change in seasons, what makes some susceptible to seasonal depression? To understand how sunlight affects our mental health, we need to first understand the pathway by which our brains use sunlight to modulate certain behaviors and hormonal processes.
Synchronizing behaviors to light/dark cycles is highly-conserved across species and is the basis for behaviors like hibernation, as well as the more prosaic sleep-wake circadian rhythm. In humans, the hormone melatonin, produced by the pineal gland, is associated with dark cycles as it is secreted during the night. Furthermore, melatonin’s secretion corresponds to the length of darkness; as the nights get longer, melatonin secretion follows suit.
Melatonin production is controlled by the suprachiasmatic nucleus (SCN) of the hypothalamus. The SCN maintains an autonomous rhythm which operates on an approximate 24-hour cycle; even outside of the body, as seen in the laboratory, SCN neurons will continue their circadian cycling.
Many studies have cited disruption in this circadian control center as a contributing factor to several mood disorders, including major depressive disorder and bipolar disorder. Even though these illnesses are not necessarily seasonal, both entail loss of consistent sleep/wake cycles as seen in SAD. One of the major forms of therapy for SAD is the use of artificial light, which alleviates symptoms in 60-80% of people. This indicates that it is not only the SCN implicated in the disease; in fact, various studies have pointed to mutations in a retinal pigment as the source.
The SCN does not only project to the pineal gland, but also has wide-ranging connections to other important areas in the brain. Therefore, if there is disruption in the SCN, the potential exists for many areas to be affected. Mood-regulating neurotransmitters such as dopamine, serotonin and norepinephrine follow circadian rhythms, and their disruption could have wide effects on mood and behavior.
Though the etiology of SAD is not fully understood, genetic research into melanopsin as well as hypothalamic genes holds promise. Until there’s a cure, the use of antidepressants, therapy, and artificial light may help SAD sufferers through the upcoming dark months.
And with holidays approaching, spending time with family, friends and good food is something we can all benefit from!
If you or anyone you know is suffering from Seasonal Affective Disorder, or any mental illness, the Penn State Milton S. Hershey Medical Center is here for you. Please follow this link for more information.