The further you live from the equator, the fewer waking hours you’ll have with the sunshine this winter. And, for those few precious hours the sun is out, it never quite reaches that overhead brightness we equate with summer fun. For some people, this is just a bummer, but for others, it actually triggers a seasonal depression. Here is some science about why it happens and some tips on how to deal with seasonal depression.

The Physiology of Winter Nights

When the sun goes down, darkness triggers the pineal gland in our brains to secrete a hormone called melatonin, whose job is to prepare us for sleep. The earlier it gets dark, the earlier that melatonin spike arrives, and the earlier we feel sleepy. Our brains release melatonin through the night to help keep us asleep. When light comes, the retina of the eyeball detects it (even through closed lids) and sends signals to the brain to slow melatonin production, allowing the brain to wake and stay alert through hours of light. This means, during the wintertime with longer nights, the brain is likely making more melatonin. That was great in times before electricity or year-round work schedules, but now, fighting it can zap our energy and leave us feeling blue. 


Seasonal affective disorder, or SAD, is a very real phenomenon, and it affects larger proportions of the population the further north you live. For example, SAD affects about 1 in 100 people living in Florida SAD, compared to about 1 in 10 living in New Hampshire [1]. 

SAD is characterized by a recurrent, seasonal occurrence of depressive symptoms such as loss of interest in the things that once made you happy, fatigue, trouble sleeping or sleeping too much, hopelessness, feeling agitated, a change in appetite, and more [1]. Some are severely incapacitated by SAD, while others notice the feelings, but are able to continue functioning through the depression. 

What Causes Seasonal Depression?

Although there is no singular, certain cause for SAD, there are several data-based hypotheses with one chemical in common: Serotonin.


We often think of serotonin as the “happy” neurotransmitter. As such, we often attribute our mental well-being, chemically, to its balance, abundance, and activity. Research has shown that those with SAD tend to express more serotonin transporter protein (SERT) in the winter than healthy individuals [2]. When more SERT is present, there is less free serotonin to bind receptors, and therefore less serotonin activity which can result in a lower mood. 


It is also possible that sunlight has some control over the maintenance of normal serotonin levels. Several studies have documented that the body produces lower levels of serotonin in wintertime, even in healthy individuals. This effect goes away when study participants get more light exposure [3].

While the mechanism of action likely occurs either directly in the skin or through a similar retinal pathway to the one that triggers melatonin production, the result is the most important finding. It means that for those whose seasonal depression is related to low serotonin, due to low light exposure, an increase in light exposure may indeed improve both serotonin production and mood.


Other findings suggest that people with SAD produce too much melatonin [1]. Overproduction of melatonin could cause an imbalance in serotonin, as serotonin is the precursor molecule. If too much serotonin is being converted to melatonin, this could also result in symptoms of low serotonin and high melatonin; namely, sleepiness and depression. 

Vitamin D

Vitamin D may also promote serotonin activity [1]. Although vitamin D is not technically an essential nutrient, as the body produces vitamin D when the skin is exposed to sunlight, vitamin D insufficiency is highly likely in the wintertime. This is generally because there is less available daylight in the winter, and cooler temperatures causing most people to cover up when they are outside. Unfortunately, this factor may further hinder serotonin activity.

How to Deal with Seasonal Depression/Wintertime Blues?

First… let there be light! We are lucky to live in a time where science-backed technology is becoming more affordable and widely available. Light therapy, specifically designed to help you deal with seasonal depression, is one such technology. Therapy lights emit sun-mimicking light without the skin-damaging UV rays. The amount of light received from a lamp is measured in “lux”, and a 20-30 minute session, sitting with your face 16-24 inches from a 10,000 lux light is a typical recommendation for those with SAD [4]. The best time to do this is first thing in the morning, waking up your brain, slowing melatonin production, and extending the number of light hours in your day. 

Unfortunately, light therapy won’t increase vitamin D production because there are no UV rays emitted by therapy lamps. However, you can make sure your vitamin D levels are adequate by taking a daily vitamin D supplement, at least during months when you know your outdoor, unprotected sun exposure is low. For many of us, due to sunscreen use and indoor work environments, sun exposure is low all year. 

Exercise is another important piece of the mood improvement puzzle. Sometimes moving our bodies is the last thing we want to do when it’s cold and dark so our workout routine takes a dive during winter months. Studies have shown that exercise can be at least as effective as antidepressant medication to relieve the symptoms of mild to moderate depression [5]. While again, the exact mechanism of this effect is not certain, scientists have noted that exercise encourages neuroplasticity, or the growth of new connections in the brain and hippocampus, and the release of stimulatory brain chemicals [6]. 

Other Nutritional Considerations

Of course, vitamin D is not the only nutrient your body needs to maintain a healthy, balanced mood. B-vitamins (especially folate) are also common culprits when it comes to vitamin insufficiency or deficiency affecting mood. B-vitamins play critical roles in the production and metabolism of neurotransmitters (including serotonin) and so a B-vitamin deficiency can easily lead to low mood and depressed feelings. If you are taking a B-complex supplement, check to make sure that your vitamins are provided in their activated forms. Namely, folate as L-methylfolate and B-12 as methylcobalamin. These forms are highly bioavailable in terms of absorption and ready-to-use without requiring much processing within the body. To learn more about factors that may be getting in the way of your healthy folate status, click here .

SAMe (S-Adenosyl-L-methionine) is yet another nutrient that occurs naturally in the body, crosses the blood-brain barrier, and stimulates both serotonin and dopamine production. It can be supplemented to boost these pathways and many others throughout the body. It should not be combined with SSRI medication, however, as it works on the same serotonergic pathways.  

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  1. NIH. “Seasonal Affective Disorder” NIMH. National Institute of Mental Health. Accessed November 2, 2020. 
  2. Willeit, Matthäus, et al. “Enhanced serotonin transporter function during depression in seasonal affective disorder.” Neuropsychopharmacology 33.7 (2008): 1503-1513.
  3. Sansone, Randy A., and Lori A. Sansone. “Sunshine, serotonin, and skin: a partial explanation for seasonal patterns in psychopathology?.” Innovations in clinical neuroscience 10.7-8 (2013): 20.
  4. Mayo Clinic. “Light therapy”. Mayo Clinic Patient Care & Health Info. Accessed November 3, 2020.
  5. Cooney, Gary M., et al. “Exercise for depression.” Cochrane database of systematic reviews 9 (2013).
  6. Poo, Mu-ming. “Neurotrophins as synaptic modulators.” Nature Reviews Neuroscience 2.1 (2001): 24-32.