Why Am I So Tired in Winter? What Blood Work Can and Can't Tell You
Last reviewed: October 21, 2026. This page is educational, not medical advice. See our medical disclaimer.
Feeling more tired in fall and winter is common. Shorter days, sleep changes, and underlying health factors can all play a part, and sometimes a seasonal pattern of depression is involved. A blood test can check a few measurable things, like iron, thyroid, vitamin D, and blood sugar. It can't diagnose poor sleep, sleep apnea, stress, or depression, including seasonal affective disorder. A normal result doesn't mean nothing is going on. This page explains common reasons winter fatigue happens and what blood work can and can't show; it is not a replacement for a clinical evaluation.
Sources include the National Institute of Mental Health, the NIH Office of Dietary Supplements, NIDDK, NHLBI, CDC, and NIST (time.gov).
Why am I so tired in winter, and is it normal?
It's common, and it rarely has one cause. The National Institute of Mental Health notes that many people feel down, or get the "winter blues," when days get shorter in fall and winter, and feel better in spring.
Tiredness usually comes from a mix of things:
- Sleep. Adults generally need 7 or more hours a night. Dark mornings and earlier evenings can quietly change when you go to bed and wake up.
- Light and routine. Less time outdoors, less movement, and a fuller calendar can all pull on your energy.
- Mood. For some people, low energy is part of a seasonal pattern of depression.
- Your body. Iron, thyroid, blood sugar, and other measurable factors can contribute to tiredness in any season.
"Common" doesn't mean you have to put up with it. If it's getting in the way of work, home, or how you feel, it's worth a conversation with a clinician.
Does the clock change on November 1, 2026 affect energy?
In 2026, daylight saving time ends on November 1 at 2 a.m. local time. When clocks fall back, mornings get lighter for a while and evenings get dark earlier.
Some people notice their sleep feels off for a while around the change. Some people notice a short-lived sleep disruption around the clock change. If tiredness continues beyond that adjustment, it is worth looking for other causes.
Winter blues or seasonal affective disorder: what's the difference?
The winter blues are a milder dip that tends to lift in spring. Seasonal affective disorder (SAD) is a type of depression with a recurring seasonal pattern. In most cases, symptoms start in late fall or early winter and go away in spring or summer. That's called winter-pattern SAD, and it can last about 4 to 5 months a year.
What it can look like
NIMH describes depression symptoms as:
- A sad, anxious, or "empty" mood most of the day, nearly every day, for at least 2 weeks
- Loss of interest in things you usually enjoy
- Decreased energy or fatigue
- Trouble concentrating
- Changes in sleep or appetite
- Thoughts of death or suicide
Winter-pattern SAD can also include oversleeping, overeating with carbohydrate cravings and weight gain, and social withdrawal, sometimes described as a "hibernating" feeling.
What else to know
A diagnosis requires seasonal episodes for at least 2 consecutive years. SAD occurs much more often in women than in men, and it's more common farther north. Winter SAD is also different from "holiday blues," which are stress-related.
NIMH discusses light therapy, psychotherapy, and antidepressant medication as treatment options for winter-pattern SAD. Vitamin D may be considered in some cases, but the evidence is less established, so it's a conversation to have with a provider. A health care provider or mental health specialist decides what fits. If this sounds like you, talk to one. If you are in crisis, call or text 988 (Suicide and Crisis Lifeline). If it's life-threatening, call 911.
What common, checkable things can contribute to tiredness?
Blood work can look at several measurable things that may relate to tiredness. None of these is a diagnosis on its own.
Iron status and blood count
Ferritin, iron, and TIBC help give a picture of your iron status. A complete blood count (CBC) looks at your blood cells. According to the NHLBI, iron-deficiency anemia is the most common type of anemia. It can cause tiredness, shortness of breath, dizziness, cold hands and feet, and pale skin. Mild or moderate cases may have no symptoms at all. Blood loss, such as heavy periods, is one risk.
Thyroid
TSH is a common first look at thyroid function. Per NIDDK, an underactive thyroid can bring fatigue, weight gain, trouble tolerating cold, dry skin, and low mood. It develops slowly. Fatigue and weight gain are also very common without a thyroid problem, so tiredness alone doesn't point to the thyroid.
Vitamin D
Your body makes vitamin D when bare skin is exposed to the sun. The NIH Office of Dietary Supplements notes that clouds, smog, older age, and darker skin reduce how much your skin makes, and that skin doesn't make vitamin D from sunlight through a window. A 25-hydroxy vitamin D test shows your current level. Whether it relates to how you feel is a question for your clinician. A low vitamin D result on its own doesn't show that vitamin D is the cause of your tiredness.
Blood sugar
Glucose, A1c, and insulin are markers a clinician may review when looking at how your body handles sugar. On their own, they don't explain tiredness.
Men with low energy, lower sex drive, and changes in strength may also want to read about low testosterone symptoms. For women, tiredness alongside changing cycles, sleep, or mood may be worth discussing in the context of menopause and perimenopause.
What can't a blood test tell you?
Quite a lot. A blood test is a snapshot of one day. It can help narrow possibilities, but it can't show sleep quality, stress, or mood. Here is how the main areas line up.
| On the Baseline Panel? | What it can (or can't) show | |
|---|---|---|
| Iron status (ferritin, iron, TIBC) | Yes | A picture of your iron status |
| Blood count (CBC) | Yes | Your blood cell counts, including signs of anemia |
| Thyroid (TSH) | Yes | A first look at thyroid function |
| Vitamin D (25-hydroxy) | Yes | Your current vitamin D level |
| Blood sugar (glucose, A1c, insulin) | Yes | Markers a clinician may review for how your body handles sugar |
| Sleep | No | Can't show poor sleep or sleep apnea |
| Stress or burnout | No | Can't measure it |
| Mood or seasonal depression | No | Not diagnosed by a blood test; needs a clinical evaluation |
Depression and SAD are diagnosed through a clinical evaluation, which can include questionnaires, as NIMH describes. A normal panel doesn't mean you're imagining how you feel. It means those particular markers weren't the answer, and that's useful too.
What else can affect winter energy?
People often ask about sleep schedule, daylight exposure, and activity when talking about winter fatigue. These factors can affect energy, but this page can't tell you which one applies in your case, and we can't promise any result. If tiredness keeps going or gets in the way of daily life, talk with a clinician. If you suspect SAD, a provider evaluation is the right next step.
When does fatigue need prompt care?
Call 911 or go to the nearest emergency room if tiredness comes with chest pain, severe shortness of breath, fainting, very heavy bleeding, or a sudden, severe headache.
Contact a clinician promptly if tiredness comes with any of these:
- Unexplained weight loss
- Fever or night sweats
- Blood in stool
- Shortness of breath that is new or getting worse
- Thoughts of self-harm
For thoughts of self-harm, call or text 988 (Suicide and Crisis Lifeline). If it's life-threatening, call 911. This list is general, not diagnostic.
Where lab work fits
It's one tool. The Refuvia Baseline Panel is $199 one time, or $349 for two. It's drawn at a local lab, and results typically arrive a few business days after the draw. Your clinician reviews your results in the context of your symptoms and history and explains them. Lab tests are not a diagnosis on their own, and a clinician looks at them alongside your symptoms and history.
If you want to know what the process looks like, see how it works. Care is provided by independent licensed clinicians with HLY Medical Group PA.
Questions about your energy?
Start a free consultation to see whether clinician-directed care through HLY Medical Group PA is appropriate for you. Refuvia does not diagnose, treat, or prescribe. Your clinician decides whether a prescription is appropriate.
Frequently asked questions
Can my blood work be normal and I still feel exhausted?
Yes. Sleep, stress, and mood all affect energy, and none of them shows up on a standard blood test. A normal panel narrows the list. It doesn't mean you're imagining it.
Does low vitamin D cause winter tiredness?
A low vitamin D result on its own doesn't show that vitamin D is the reason you're tired, and a lab result alone isn't a diagnosis. A 25-hydroxy vitamin D test shows your current level, and a clinician can help decide whether it matters for how you feel and what, if anything, to do about it.
How long does winter-pattern SAD last?
Symptoms typically start in late fall or early winter and last about 4 to 5 months a year, easing in spring or summer. A diagnosis requires seasonal episodes for at least 2 consecutive years.
Are holiday blues the same as SAD?
No. Holiday blues are stress-related, while SAD is a type of depression with a recurring seasonal pattern. If low mood or fatigue lasts or keeps returning, a provider can help sort out which it is.
Does being tired mean my thyroid is off?
Not necessarily. Fatigue is a common symptom of an underactive thyroid, but it's also very common without a thyroid problem. A TSH test is a first look at thyroid function, and a lab result alone isn't a diagnosis. A clinician looks at it alongside your symptoms and history.
Why do some people feel the winter slump more than others?
It varies. SAD occurs much more often in women than men and is more common farther north. Sleep, health history, and stress also differ from person to person.