The clocks went back on Sunday 25 October. For a few days it feels like a gift, an extra hour in bed, and then the reality lands: it is dark by the time you leave work, the mornings are grey, and by mid-afternoon you are running on fumes.

Feeling more tired in winter is common, and a good deal of it is simply the season. But tiredness, low mood and feeling the cold are also the classic features of several common, treatable conditions, and “it’s just winter” is the perfect cover for them. This guide helps you tell the two apart: what winter genuinely does to you, the four blood checks most worth considering at this time of year, the clues that point beyond the season, and the causes no blood test will find.

It is the seasonal companion to our main article on being always tired no matter how much you sleep, which covers the full fatigue work-up. A blood test is information, not a diagnosis: results need to be read alongside your symptoms and history.

What winter genuinely does to you

Some of the winter slump is real, and none of it needs a blood test to explain:

  • Less daylight. The NHS says seasonal depression is thought to be linked to reduced winter daylight, which may affect brain chemicals such as melatonin and serotonin that influence mood and sleep patterns. Dark mornings and short days can leave you groggy.
  • Changed routines. Less time outdoors and less exercise lower energy, and it becomes a cycle: you feel tired, so you move less, so you feel more tired.
  • Seasonal low mood. Seasonal affective disorder (SAD) is depression that returns each winter and goes away in the summer. The NHS lists low mood, loss of interest, increased tiredness and sleeping more among its symptoms, and a diagnosis is usually made when it has happened at the same time of year for two years or longer. It is diagnosed by a doctor, not a blood test. Our guide to winter blues and SAD covers what a test can and cannot do.
  • Winter bugs. A run of colds, or a slow recovery from one, can leave you drained for weeks. If you seem to catch everything going, see why you might keep catching colds.
  • The festive run. Late nights, more alcohol and richer food through December add up. Our after-Christmas blood test guide covers that stretch.

The difficulty is that these overlap with the medical causes below, and a deficiency can go unnoticed for months because each winter you put it down to the weather.

Clues that it is more than winter

Seasonal tiredness tends to follow a pattern: it builds as the days shorten, eases on bright days and at weekends, and lifts in spring. It is worth looking further if:

  • your symptoms have lasted more than a few weeks and are not improving (the NHS advises seeing a GP if you have felt tired for a few weeks and do not know why)
  • this winter feels noticeably worse than previous ones, or last winter’s symptoms never fully lifted
  • tiredness started well before the clocks changed
  • you are breathless on mild exertion, your heart seems to pound, or you look paler than usual
  • you have pins and needles, a sore tongue or mouth ulcers
  • you feel cold when others in the same room are comfortable, with weight gain, constipation or dry skin
  • several symptoms have arrived together, such as tiredness, low mood, hair shedding or poor concentration
  • a week off, a few good nights’ sleep or a bright day make no difference

If several of these apply, checking now is more useful than waiting for spring to see whether it improves.

The four checks worth considering this winter

CheckWhy it matters in winterMain blood test
Vitamin DUK sunlight is too weak to make it from October to early March25-OH vitamin D
Iron storesOften first noticed as “winter” tirednessFerritin with a full blood count
Vitamin B12Vague symptoms that are easy to blame on the seasonVitamin B12
Thyroid functionAn underactive thyroid looks like a bad winter and does not lift in springTSH and free T4

1. Vitamin D

This is the one most directly tied to the season. The NHS says that between October and early March we do not make enough vitamin D from sunlight, and that everyone should consider a daily supplement containing 10 micrograms (400 IU) during autumn and winter. That advice applies whether or not you have had a test.

On the standard 25-OH vitamin D test, UK clinical guidance treats a result below 25 nmol/L as deficient, 25 to 50 nmol/L as possibly inadequate for some people, and above 50 nmol/L as sufficient for most people. Deficiency can cause bone pain and, in adults, a bone condition called osteomalacia. Tiredness and aching muscles are sometimes reported with low levels, but they are non-specific, and the UK recommendations are based on bone and muscle health rather than energy or mood, so a low result should not be assumed to explain how you feel.

Because levels built up over summer tend to fall through the winter, a test in autumn shows roughly your starting point and a test in late winter shows your low point. UK clinical guidance does not recommend routine vitamin D testing for people without symptoms or a condition that makes it worthwhile, so on the NHS a test is usually reserved for those groups. Some people who simply want a number choose a private test. For the detail see our guides to why everyone in the UK should think about their vitamin D in winter, including the safe limits on supplements, and to vitamin D deficiency symptoms.

2. Iron and ferritin

Iron deficiency is not seasonal, but winter is when many people first notice it. The NHS lists tiredness and lack of energy, shortness of breath, noticeable heartbeats, paler skin and headaches as common symptoms of iron deficiency anaemia, with hair loss and nail changes less common. Heavy periods and pregnancy are very common causes, and blood loss from the gut is another, which needs a doctor’s assessment.

Ferritin reflects your iron stores. British Society of Gastroenterology guidance says a ferritin below 30 µg/L is generally indicative of low iron stores, that most laboratories’ lower limit of normal lies between 15 and 30 µg/L, and that ferritin can look normal in someone who is iron deficient when there is inflammation. Iron stores can run low before anaemia shows on a basic full blood count. Risk factors include heavy periods, blood donation, a diet low in iron, long-term anti-inflammatory painkillers or acid-suppressing medicines, and endurance sport. If your iron is low, finding out why matters, so please speak to your GP before starting iron tablets. Our guide to what low ferritin feels like covers the full picture.

3. Vitamin B12

B12 is needed for healthy red blood cells and nerves. The NHS lists extreme tiredness, a lack of energy, pins and needles, a sore red tongue, mouth ulcers, and problems with memory and concentration as possible signs. Pernicious anaemia is the most common cause in the UK. A vegan diet without supplements or fortified foods, and some medicines, including proton pump inhibitors and metformin, can also lower B12. The NHS says about 1 in 20 people aged 65 to 74 and 1 in 10 aged 75 or over are affected.

For interpretation, NICE guideline NG239 treats a total B12 below 180 ng/L as confirming deficiency, 180 to 350 ng/L as indeterminate, and above 350 ng/L as making deficiency unlikely, or the laboratory’s own thresholds where they differ. If you have pins and needles, see your GP promptly: the NHS warns that some of the problems caused by deficiency can be irreversible if it is not treated early.

4. Thyroid function

An underactive thyroid builds slowly. The NHS lists extreme tiredness, feeling colder than usual, weight gain, constipation, trouble concentrating, low mood, dry skin and hair loss among its symptoms, which usually develop slowly and get worse over time. In winter that is easy to dismiss, because everyone is cold. Unlike seasonal tiredness, it does not ease when the days get longer. If you are the person in the office who needs a jumper when others do not, it is worth checking.

NICE guideline NG145 says TSH is the usual first test for adults, with free T4 added on the same sample if TSH is above the reference range. See our thyroid blood test page, our guide to underactive and overactive thyroid tests, and why am I always cold? for the link between the thyroid and feeling the cold. An overactive thyroid can also cause tiredness, with restlessness, anxiety and muscle weakness.

Hormones can look like winter too

Tiredness, low mood and poor sleep in your 40s can be part of perimenopause. NICE guideline NG23 says that in otherwise healthy women aged 45 or over with symptoms, perimenopause is identified from the symptoms, without a blood test. In men, tiredness, low mood and low libido can be signs of low testosterone, though these symptoms are non-specific and many men who have them have normal levels.

What a blood test cannot tell you

A clean result does not mean your tiredness is imagined. It means the common deficiencies are unlikely to be driving it, and the focus can move to the things blood cannot measure:

  • Daylight. The NHS recommends getting outside in daylight every day for seasonal low mood. Morning light is the easiest to fit in.
  • Movement. The NHS advises regular exercise and a healthy diet for tiredness, even when it is the last thing you feel like doing.
  • Sleep routine. The NHS advises regular sleep and wake times, a dark, quiet, cool bedroom, and avoiding screens in the last hour before bed.
  • Alcohol. The NHS advises not drinking too much, which matters more in the party season.
  • Sleep apnoea and other conditions. The NHS lists sleep apnoea, diabetes, ME/CFS and infections such as glandular fever and COVID-19 among the causes of tiredness. Several are assessed by a clinician rather than a single blood test.
  • Mood. If low mood is the main problem, speak to your GP. Talking therapies, light therapy and antidepressants are all used, and SAD is treatable.

When to see your GP or seek urgent care

See your GP rather than relying on a private test if your tiredness comes with unexplained weight loss, night sweats, a new lump that does not go away, blood in your stool or urine, a fever that keeps returning, or pins and needles. Call 999 or go to A&E for chest pain, severe breathlessness or collapse. Use NHS 111 if you are worried and it is not an emergency. If you are feeling very low or having thoughts of harming yourself, call NHS 111 or Samaritans free on 116 123 at any time, or call 999 if you may be in immediate danger.

Getting tested at WMG Health

Our General Wellness panel (£279) covers all four checks in one fixed-price test: a full blood count, iron studies including ferritin, vitamin D and B12, thyroid function (TSH, FT3, FT4), CRP and ESR, and zinc. It also includes folate, HbA1c, liver and kidney function and a lipid profile (cholesterol). If your tiredness comes with hair shedding, Hair Loss Essentials (£249) covers ferritin and iron studies, vitamin D, B12, folate, thyroid function, a full blood count, zinc and CRP. Other markers can be added through a custom panel, with the price confirmed before booking. For hormones, see The Hormone Specialist (£299), which does not include iron, vitamin D or B12.

Your blood is drawn at 134 Harley Street, with no GP referral needed. Results are released as each marker is reviewed by a GMC-registered doctor, with the expected timeframe confirmed at booking, and anything that needs follow-up is flagged.

Common questions

Is it normal to feel more tired in winter? To a degree, yes. Less daylight, less activity and seasonal mood changes make many people feel more sluggish from October to March. It is worth checking if the tiredness is severe, lasts weeks, comes with other symptoms, or does not lift in spring.

When is the best time to test vitamin D? Any time is useful. Testing in autumn shows your starting point for winter, and testing in late winter shows your low point.

Should I just take a vitamin D supplement instead? The NHS advises everyone to consider 10 micrograms a day in autumn and winter, whether or not you test. A test is useful if you have symptoms, are in a higher-risk group, or are considering a higher dose.

Do I need to fast? Yes, for these panels. The General Wellness and Hair Loss Essentials panels both need a fast of at least 12 hours beforehand: water is fine, and encouraged, but no food or other drinks. Do not take iron tablets for 24 hours or a B12 supplement beforehand, and if you take folate, B-complex or multivitamin supplements, hold them for 24 to 72 hours, so the result reflects your true level. Your booking confirmation gives the instructions for your panel.

Can a blood test tell me if my low mood is SAD? No. SAD is diagnosed by a doctor, usually from your pattern of mood over time. A blood test can only look for physical conditions that cause similar symptoms.

My results were normal but I still feel exhausted. What next? A normal panel is reassuring about the markers tested, but it does not rule out every cause. Our guide to being always tired no matter how much you sleep covers the wider causes, and your GP can look at sleep problems and mood.

  • NHS or Private Blood Test? How to Choose: when an NHS test is the right first step and when a private test makes sense.
  • How Often Should You Have a Health Check?: age-by-age guidance on health checks and blood tests.

Sources

  • NHS: Tiredness and fatigue; Seasonal affective disorder (SAD); Vitamin D; Iron deficiency anaemia; Vitamin B12 or folate deficiency anaemia; Underactive thyroid (hypothyroidism). nhs.uk
  • NICE Guideline NG145: Thyroid disease: assessment and management. nice.org.uk
  • NICE Guideline NG239: Vitamin B12 deficiency in over 16s: diagnosis and management. nice.org.uk
  • NICE Guideline NG23: Menopause: identification and management. nice.org.uk
  • British Society of Gastroenterology: Guidelines for the management of iron deficiency anaemia in adults (Snook et al, Gut 2021). bsg.org.uk
  • Royal Osteoporosis Society: Vitamin D and bone health: a practical clinical guideline for patient management. theros.org.uk
  • Scientific Advisory Committee on Nutrition (SACN): Vitamin D and health (2016). gov.uk

This article is for general information only and does not constitute clinical advice. If you have concerns about your symptoms, book an appointment or speak to your GP.