Maybe you are snapping at things that did not used to bother you. Maybe sleep feels a little harder to settle into. Maybe your energy has a ceiling it did not have in the summer, and you are catching every cold that comes through. You have not changed anything. So what is going on?
There is one thing that connects all of those symptoms -- and it starts declining before you feel it.
Vitamin D does not drop suddenly. It declines gradually, across weeks and months, as the hours of daylight shorten. By the time you notice you feel different, the drop has already been happening for a while. And because it happens slowly, most people write it off as stress, or getting older, or just the season.
It is not just the season. It is your vitamin D, and it started declining before you felt it.
What Vitamin D Actually Does in Your Body
Vitamin D is not really a vitamin. It functions more like a hormone. It regulates activity in nearly every tissue in your body -- your brain, your immune system, your muscles, your bones, and the pathways involved in mood and sleep.
Sleep is one of the first places people notice a change. Vitamin D plays a role in melatonin production -- the hormone that signals your brain to wind down at night. When vitamin D is low, that signal is weaker. You lie in bed feeling like you should be tired but cannot quite get there, or you wake up earlier than you want to and cannot get back to sleep.
Mood is another. Vitamin D receptors are found in the brain regions that regulate dopamine -- the neurotransmitter responsible for motivation, pleasure, and satisfaction. When vitamin D is low, dopamine signaling is impaired. A shorter fuse, less enjoyment of things that used to feel good, low motivation, irritability that does not match the circumstances. In my experience, anger that seems out of proportion is often low dopamine -- and low vitamin D is frequently what is driving it.
Immune function is the third piece. Vitamin D receptors are present on immune cells and vitamin D plays a direct role in how the immune system mounts a response. When levels are low, the immune system is slower and less precise. If you are getting sick more easily, or taking longer to recover, low vitamin D is one of the first things I look at.
If fatigue is part of your picture, this is one of the first things worth ruling out -- you can read more about how I approach it at the Wellness Architecture fatigue page.
A Story That Made It Very Clear
Years ago, I had a patient who had moved to Seattle from a sunnier climate. She was tossing and turning all night, waking up achy, and was noticeably grouchier than she had ever been. She was not connecting any of it to the weather or the light -- she just felt off and could not understand why.
At some point she started going to a tanning booth a couple of times a week. Almost immediately, her sleep improved dramatically. Her mood lifted. The achiness faded. She felt like herself again.
When she told me about it, it was immediately clear what had happened. The UV exposure was triggering vitamin D production -- the same production her body had been making effortlessly when she lived somewhere with real sun. Two sessions a week was enough to turn it around.
The tanning booth is not the recommendation. But that case made the mechanism impossible to ignore. Light, vitamin D, dopamine, mood, sleep -- they are all connected, and when the light goes away, the chain reaction is real.
Why the Bay Area Does Not Protect You
We live in California. More sun than almost anywhere else in the country. So vitamin D deficiency should not be a problem here, right?
It is. I see it constantly in labs.
Sun protection is one reason -- most women in this age group are wearing SPF daily, which blocks the UV rays needed for vitamin D production. The angle of the sun is another: from October through March, the sun in the Bay Area is too low in the sky for most of the day to produce meaningful vitamin D even from unprotected skin. Women who are exercising outdoors, living in California, doing everything right -- and their vitamin D is nowhere near where it needs to be.
The "Normal" Lab Range Does Not Mean Optimal
Most doctors check vitamin D and tell you that you are fine if you are above the laboratory minimum -- typically around 20 ng/mL. But here is the problem with that number: lab reference ranges are built from population averages. As the overall population gets sicker and more deficient, the average shifts down -- and the "normal" range shifts with it. The threshold is not telling you what optimal looks like. It is telling you where the average of an already-deficient population sits.
Clinically, most women feel and function noticeably better with levels between 75 and 85 ng/mL. Below that, even without flagging as officially deficient, the effects of low vitamin D are already showing up -- in sleep, in mood, in energy, in how well the immune system responds.
If you have been told your vitamin D is normal and you still feel the way this blog describes, your number is worth a second look.
Why Now Is the Time to Act
Most people think about this after the fact -- after the holidays, when they have been dragging through the darkest months and cannot figure out why. By that point, levels have been low for the entire fall. It takes time to bring them back up.
The better approach is to get ahead of it now, before the clocks change and before levels hit their winter floor. The right dose depends on where you are starting, so knowing your actual number matters.
Finding the Right Support
If your energy, sleep, mood, or immune resilience has been worse than usual and you cannot point to a reason, vitamin D is one of the first things worth checking. It is a simple test, and the results change what you do.
If you are a patient of mine, we look at this as part of your regular fall check-in. If you are not, a Discovery Call is the place to start.
Frequently Asked Questions
How do I know if my vitamin D is low?
A blood test -- the 25-OH vitamin D test. Most primary care doctors can order it. The key is not just whether your number is above the lab minimum, but whether it is in the range where vitamin D actually does its job. If you have been told you are normal and still feel the way this blog describes, your number is worth a closer look.
Can I just take a standard vitamin D supplement from the store?
You can, but the dose matters. Most over-the-counter supplements are not enough to meaningfully raise levels in someone who is already low. Vitamin D3 is the form to use, taken with a meal that contains fat. Knowing your starting level tells you whether what you are taking is actually working.
Does vitamin D affect hormones?
Yes, though that is a deeper conversation. The most direct effects are on energy, sleep, immune function, and mood through the dopamine and melatonin pathways. There are interactions with thyroid function and estrogen metabolism as well, but if hormones are your primary concern, that warrants its own investigation.
I feel fine right now. Do I still need to worry about this?
Vitamin D levels in Northern California typically reach their lowest point in January and February. If you start addressing it in October, you have a real head start. If you wait until you feel terrible, you are already months behind.
Related reading:
- You Are Not Losing Your Mind. Your Brain Is Running on an Empty Tank.
- She Had Not Slept Through the Night in 15 Years. Here Is What It Actually Took.
- You Are on Hormones and Still Don't Feel Like Yourself. Here Is What Nobody Has Looked At.
For more on vitamin D and menopause, the Menopause Society provides current patient resources.