Many individuals struggle with low mood and lack of motivation during the late-autumn and winter months. For some, these symptoms may be indistinguishable from depression. When this happens, the condition is known as seasonal depression or seasonal affective disorder (appropriately named SAD).

One may think that the reasons why these feelings tend to emerge this time of year are largely psychological: The holidays have come and gone, it may be unpleasantly cold outside, and one may find themselves socializing less due to inclement weather. What may be surprising to find is that SAD does not merely arise because of these conditions. There are biophysiological reasons for it, as well.

When the body is exposed to too little sunlight, as is the case in the winter months, this can cause anomalies in certain hormone and neurotransmitter levels. For example, in some individuals, a lack of adequate sunlight causes the body to dial up the production of SERT, a protein that is responsible for transporting the neurotransmitter serotonin in and out of the brain. As SERT production increases, the amount of serotonin available in the brain decreases, and low levels of serotonin are strongly associated with depressive symptoms.

Apart from being vital to mood regulation, serotonin is also one of the neurotransmitters responsible for maintaining a healthy sleep-wake cycle. As the name suggests, this sleep-wake cycle (also known as one’s circadian rhythm) is an internal clock that tells the body when it is time to go to sleep and when it is time to wake up. Over the course of a 24-hour cycle, hormone and neurotransmitter levels oscillate depending on the time of day. Some make us alert while others cause relaxation. As the day begins, levels of the former are at their peak and tell our brain that it is time to wake up and get moving. They then decline as the day progresses, while the latter increase and finally reach their peak early in the evening. This ensures that we go to sleep at a decent hour so that we can be awake the next day feeling refreshed.
Predictably, when the balance of hormones and neurotransmitters that regulate circadian rhythms are disrupted, this translates into poor-quality sleep. Less predictably, there is a strong correlation between disruptions in circadian rhythms and numerous conditions that affect mental health—not just SAD. Some even posit that the relationship between sleep disturbances and major depression is so strong that a diagnosis of depression without the presence of sleep problems should only be made in extenuating circumstances.

Sleep Problems and Depression

As discussed above (and as I’ve written about this previously), the link between sleep, which is regulated by an individual’s circadian rhythm, and depression is well established. What is perhaps more important is that the relationship between the two appears to be bidirectional—one may worsen the other, because the underlying mechanisms for both conditions are similar.

In an individual predisposed to depression, one of the biophysiological mechanisms may include imbalances in neurotransmitter levels, which may disrupt the circadian rhythm. Conversely, individuals who are predisposed to disruptions in circadian rhythms during the winter months may experience imbalances in neurotransmitter levels, thereby leading to depression.

This link may have implications that extend well beyond the treatment of SAD. While evidence has shown that therapies that focus on resetting the circadian rhythms of patients with SAD can improve sleep quality and mitigate depressive symptoms, the same can be said of patients with non-seasonal depression. In summary, improving sleep quality can effectively treat various types of depressive disorders and consequently sleep quality can be improved by correcting the circadian rhythm. One of the best ways to accomplish this is through light therapy.

What Is Light Therapy?

Light therapy is exactly what it sounds like. Clinicians use lamps to expose patients to artificial light that mimics natural light. These are typically extremely bright lights that produce between 5,000 and 10,000 lumens. By comparison, a 100-watt incandescent bulb produces approximately 1,600 lumens. Typically, light therapy sessions last between thirty minutes to an hour early in the morning when the hormones that make us rise and shine are at their peak, which helps to reset the body’s internal pacemaker.

Evidence has long suggested that abnormalities in circadian rhythms can be successfully treated with light therapy. This includes minor disruptions caused by jetlag or sudden shifts in work schedules, as well as more significant disruptions like those associated with SAD. Additionally, research has indicated that light therapy may enhance cognitive performance and mitigate some symptoms associated with dementia.

Apart from treating these conditions, light therapy is considered generally safe (though it has been shown to cause bipolar patients to switch from the depressive phase to manic phase) and typically cause mild side effects, some of which include:

•    Eyestrain
•    Headache
•    Irritability
•    Nausea

Light Therapy for Conditions Beyond SAD

A Dutch study published in 2013 found that just 30 minutes of light therapy per day for three consecutive weeks was enough to significantly reduce symptoms of SAD. Perhaps even more importantly, the study also included patients with non-seasonal forms of depression though the results were similar: Light therapy ameliorated depressive symptoms in both types of depression.

As the disruption of circadian rhythms is a feature of both SAD and non-seasonal depression, correcting one’s sleep-wake cycle appears to be of paramount importance when treating depression. This includes not only SAD and non-seasonal depression, but also bipolar disorder, premenstrual dysphoric disorder, and both ante-partum and post-partum depression. (Light therapy has the potential to be extremely useful for the latter two groups, since pregnant or nursing women may prefer not to take antidepressants.)

Light therapy has also been used to effectively treat conditions besides depression. This includes adult attention deficit (hyperactivity) disorder (ADHD) and even some forms of dementia (such as Alzheimer’s disease), as sleep disruptions and abnormal sleep pattens are key features of both conditions. More than 70% of children and adults with ADHD report insomnia, while lack of quality sleep is common among the elderly and has been associated with Alzheimer’s disease and cognitive decline in general.

Ultimately, the clinical benefits of light therapy reveal just how important sleep and a properly attuned circadian rhythm are for mental health. It is certainly not a silver bullet by any means, but it is generally safe, inexpensive, and it may prove to be more than just a treatment for the winter blues.

Dr. Ahmad reports no conflict of interest. He is not a speaker, advisor, or consultant and has no financial or commercial relationship with any biopharmaceutical entity whose product/device may have been mentioned in this article.