Now it is November, the clocks have changed and for many it brings darkness and rain for months. For some of us, it also brings Seasonal Affective Disorder (SAD). Whether or not symptoms of SAD eclipse your normal disposition, there is a good chance the sun’s absence this time of year will affect four key areas of your health.
Vitamin D: The Sunshine Hormone
Ultraviolet rays from the sun catalyse the synthesis of vitamin D on contact in the human skin, and to a smaller degree we consume it in fish, eggs and mushrooms. This little hormone carries some big distinctions. It has been studied extensively for its roles in modulating the immune system, insulin sensitivity, neurotransmitter synthesis, calcium uptake and bone resorption, and may have a place in preventing devastating diseases like cancer and multiple sclerosis. The vitamin D receptor (VDR), present in almost every tissue of the body, ushers in vitamin D so it can enhance gene transcription at the vitamin D response element (VDRE) on the DNA itself.
What Contributes to Lower Vitamin D Status in Winter?
- Low production: With fewer daylight hours and cold temperatures, the likelihood of getting enough vitamin D from sunshine diminishes. For those with more melanin pigment in the skin, expect even lower production during this time.
- Low intake: Vitamin D is fat-soluble, so its bioavailability depends on proper administration. Taking fat-soluble vitamins with a meal helps absorption. Cod liver oil, cholecalciferol supplements, and fatty fish like tuna, mackerel, sardines and salmon all help raise circulating vitamin D levels.
- High cortisol: High cortisol and its pharmaceutical analogues are known to reduce VDR expression and thus limit vitamin D uptake and activity in the body. People with high cortisol levels, especially during the winter vitamin D famine, may want to take extra precautions.
Our Vitamin D 25-OH Total Blood Spot Test measures both D2 and D3 from a simple finger-prick at home — ideal for checking your levels as the darker months set in.
Vitamin D and the Serotonin Seesaw
If you don’t have enough vitamin D to bind VDREs, serotonin synthesis plummets — and people really feel the domino effect of that drop in serotonin.
An important reason why the discussion of SAD focuses so much on vitamin D is because the VDREs present in the brain, when bound by vitamin D, act to up-regulate the synthesis of 5-HTP — the precursor to serotonin. Serotonin acts as a modulator of inflammation and the allergic response, as a signal for almost every type of immune cell, in gut function, and in the central nervous system as an inhibitory neurotransmitter that gives us emotional flexibility.
In order to make serotonin anywhere in the body, the enzyme tryptophan hydroxylase (TRPH) must transform tryptophan (an amino acid obtained from protein) into 5-HTP. Because vitamin D binding to the VDREs in the brain increases the activity of the TRPH2 enzyme there, you get more serotonin. If you do not have enough vitamin D to bind those VDREs, serotonin synthesis plummets and some people really feel the domino effect of that drop.
In the absence of vitamin D, the activity of TRPH1 (the non-neuronal form) responsible for peripheral serotonin production is enhanced — in contrast to the brain TRPH2. This is why peripheral serotonin levels measured in the urine may look normal or high while brain levels are simultaneously quite low. Knowing vitamin D status therefore colours our understanding of potential serotonin levels in different compartments of the body. If you are experiencing low mood, anxiety or depression during the winter months and want to assess your serotonin, dopamine and other key neurotransmitters directly, our Neurotransmitter Testing (Dried Urine Spot) measures serotonin, dopamine, GABA, norepinephrine and other key brain chemicals — providing a biochemical picture of the imbalances that may be driving your seasonal symptoms.
The Cortisol Awakening Response in SAD
Sunlight influences the diurnal rhythm and when it becomes scarce, those rhythms can become dysregulated. Cortisol follows a very typical diurnal rhythm — normally, as the sun begins its ascent in the morning, cortisol charts its course toward the highest level of the 24-hour day. Within 30 minutes of waking, cortisol should continue to rise by about 50% from the waking level. This is called the Cortisol Awakening Response (CAR).
When the mornings are dark, in susceptible individuals the diurnal rhythm of cortisol flattens out and it becomes difficult to shake off sleep and function normally. The defect in cortisol regulation in SAD may be missed entirely if the CAR is not assessed alongside the diurnal rhythm.
Our Adrenal Stress Profile (Saliva) measures cortisol at four time points across the day alongside DHEA-S — capturing the diurnal cortisol curve and helping to identify whether a blunted CAR may be contributing to your winter fatigue and low mood.
Altered Melatonin Rhythm in SAD
Darkness signals the nightly melatonin flood in the central nervous system. With darkness creeping in more and more during autumn and winter, we naturally find more and prolonged central melatonin production. Some researchers theorise that the increased melatonin production during winter steals tryptophan from other parts of the brain that normally use it to build serotonin — leaving those parts of the brain at a serotonin deficit. Put this possible tryptophan-steal scenario together with a vitamin D deficiency, and it is easy to see how a neural serotonin imbalance propagates.
In urine, the first morning void contains the melatonin metabolite (MT6s) that has accumulated in the bladder during sleep, so the highest levels of the day are expected in that first morning result. In a patient with SAD, MT6s may remain at the overnight level instead of dropping significantly after awakening — a groggy start to the morning with continued night-time melatonin production on board. Our Sleep Balance Profile Test measures melatonin (MT6s) alongside cortisol across the day — making it the ideal test for identifying the hormonal pattern associated with SAD and seasonal sleep disruption.
Since the absence of light has such an effect on the body clock, natural light therapy has been studied extensively for helping combat SAD — used to clear out morning melatonin, stimulate cortisol production and improve mood overall.
Preparing for the Darker Months
The good news is that the dark days of winter can give us a reason to turn inward, get the sleep we desperately need after a busy summer, and gather up energy for the season ahead. With the right testing and targeted support — whether vitamin D supplementation, light therapy or hormone balancing — SAD does not have to define your winter.
Originally ZRT Laboratory. Reproduced with permission. Last reviewed: May 2026.