Seasonal affective disorder (SAD) is a form of depression that occurs seasonally, typically ramping up in the fall and winter months and disappearing come spring.^1,2,3It’s been estimated⁴ that as many as 20 percent of Americans are affected by SAD each winter.

What differentiates SAD from regular depression is that a full remission occurs in the spring and summer months. Common SAD symptoms include oversleeping, intense carbohydrate cravings, overeating and weight gain. Some people also have trouble concentrating and withdraw socially, preferring to “hibernate” indoors instead of carrying on with their normal day-to-day activities.⁵

Dr. Norman Rosenthal, clinical professor of psychiatry at Georgetown University School of Medicine, was the first to describe SAD, writing in a 1984 journal article⁶ that the “depressions were generally characterized by hypersomnia, overeating and carbohydrate craving, and seemed to respond to changes in climate and latitude.”

Indeed, rates of SAD vary depending on location, with people living farthest from the equator in northern latitudes being most susceptible. In the U.S., for instance, SAD affects just 1 to 1.4 percent of Floridians compared to 9.7 percent of people living in New Hampshire⁷ and 9 percent of Alaskans.⁸

The Importance of Vitamin D

Many have become familiar with the importance of sun exposure for optimizing your vitamin D level,⁹ and there is research showing that not only is SAD more common in people with low vitamin D, but raising your level also improves symptoms of SAD.^10,11

Your body produces vitamin D through exposure to UVB light. Unfortunately, for those living in northern latitudes, this may only be an option for a few short months each year. As a general rule, you have to live below 22 degrees latitude if you want to produce any vitamin D at all during the winter. The charts below display the likelihood of vitamin D synthesis across the U.S. by month.

While supplementation may be required during months when you cannot produce sufficient amounts through sun exposure, it’s important to get your vitamin D level tested before you start taking supplemental vitamin D. This will help you fine-tune your dosage over time.

For optimal health, including mental health, you’ll want a vitamin D level between 60 and 80 ng/mL¹² (150 to 200 nmol/L), with 40 ng/mL being the lower cutoff for sufficiency. GrassrootsHealth has an online vitamin D calculator that can help you estimate the oral dosage required to get you into a healthy range, based on your starting blood level.

Considering vitamin D’s impact on mental health and brain function, it would certainly be prudent to make sure your vitamin D level is optimized if you struggle with SAD. Vitamin D is only part of the equation, however, as sunlight influences your mood in general, and SAD specifically, in other ways as well.

SAD Linked to Circadian Rhythm Disruption

Your body’s circadian rhythm, which directs a whole host of physiological processes, is calibrated by exposure to natural sunlight and darkness. When sunlight is lacking, it can disrupt your circadian rhythm, triggering SAD.

For instance, sunlight helps keep your level of the protein SERT low. As a key player in transporting the neurotransmitter serotonin, known to play a role in mood, having a low level is a good thing, as higher SERT levels are linked to lower serotonin activity and increased depression. As noted in the journal Depression Research and Treatment:¹³

“In one study,¹⁴ people with SAD had 5 percent more SERT, a protein that assists with serotonin transport, in the winter months than in summer … Throughout the summer, sunlight generally keeps SERT levels naturally low. But as sunlight diminishes in the fall, a corresponding decrease in serotonin activity also occurs.”

Melatonin is another important player that is influenced by your circadian clock and exposure to bright full-spectrum light. Your master biological clock resides inside the suprachiasmatic nucleus (SCN) of your brain, which is part of your hypothalamus. Based on signals of light and darkness, your SCN tells your pineal gland when it’s time to secrete melatonin and when to turn it off. One of melatonin’s primary roles is regulating your body’s circadian rhythm.

When it gets dark, your brain starts secreting melatonin (typically around 9 or 10 p.m.), which makes you sleepy. Levels typically stay elevated for about 12 hours; then, as the sun rises, your pineal gland reduces your production and the levels in your blood decrease until they’re hardly measurable.

In people with SAD, melatonin production appears to be disturbed. Some may overproduce melatonin, leading to feelings of sleepiness and lethargy. In others, melatonin production may be phase-delayed, which means it’s produced at the wrong time.¹⁵

The combination of low serotonin and excess melatonin may prove to be especially problematic for your circadian rhythm, and there’s evidence that, for people with SAD, “the circadian signal that indicates a seasonal change in day length has been found to be timed differently, thus making it more difficult for their bodies to adjust.”¹⁶