The days are beginning to get shorter and we’re spending more time indoors as most of the day’s sunlit hours are occurring during the work day. For our bodies, this is a significant change to adjust to, especially for those of us who live in cooler climates. It’s common to experience some decline in mood during the winter – feelings of anxiety and depression are known to be more prevalent throughout the colder months. For some individuals, these seasonal mood changes along with fatigue, irritability, and appetite changes are especially pronounced, and this is called Seasonal Affective Disorder (SAD). Incidence of SAD increases with distance from the equator, and light therapy is an effective treatment for many individuals with SAD. Reduced sunlight exposure, resulting in shifts in circadian rhythms and alterations in serotonin metabolism, is thought to underlie this condition.
Timing of exposure of the retina to light affects the sleep-wake cycle, and inadequate light exposure during the winter disrupts this cycle. Light therapy independent of vitamin D production (ultraviolet light is filtered out), especially in the morning hours, is known to be effective for treating SAD and major depression. SAD has been successfully treated with light therapy since the early 1980s. For depression, a meta-analysis in 2005 revealed that the effectiveness of light therapy was comparable to that reported in many trials of anti-depressant drugs. Light entry into the retina inhibits production of melatonin, a hormone that promotes sleep. Light therapy helps to restore melatonin, the neurotransmitter serotonin, and other mood-regulating molecules to their normal time cycles and levels of production; consequently depressive symptoms are reduced.
Reduced exposure to sunlight during the winter also means less natural vitamin D production by the skin, , and vitamin D itself may also regulate mood. There is still much unknown about how vitamin D works in the brain, but there are plenty of vitamin D receptors in the brain, and it is thought to affect daily biorhythms and production of neurotransmitters, including serotonin. Also, low circulating vitamin D is associated with SAD and major depression. So far, only a handful trials of vitamin D supplementation for seasonal depressive symptoms have been performed, and some of these used doses that were likely too low to have any measurable effect (400 IU; currently many experts believe that 2000 IU/day or more may be necessary for most people to maintain adequate blood 25(OH)D levels ). A dose of 800 IU improved mood of healthy subjects during winter in one trial , but had no effect in another trial. A third trial used a dose of 4000 IU/day for six months starting in the summer, and then evaluated feelings of wellbeing during the December-February time period. The 4000 IU dose of vitamin D produced an average 25(OH)D level of 45 ng/ml and improved wellbeing scores compared to a 600 IU dose (average 25(OH)D level of 32 ng/ml). 
Current research is investigating the connection between depression and diabetes with a focus on vitamin D. Because depression is associated with insulin resistance, and vitamin D is thought to affect insulin secretion by the pancreas, ongoing studies are evaluating whether vitamin D supplementation can help to prevent diabetes.  In fact, healthy adults with low vitamin D levels were more likely to develop diabetes within 10 years compared those with adequate levels, according to a recent study. 
Vitamin D is active in essentially every cell and tissue in the human body. It is crucial (for everyone, not just sufferers of SAD ) to maintain adequate 25(OH)D levels (Dr. Fuhrman recommends 35-55 ng/ml) with supplementation. Especially if you note mood or sleep issues during the winter, be sure to accompany vitamin D supplementation with plenty of morning light.
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