Why am I so tired in winter? When seasonal fatigue could mean something more

Why am I more tired in winter?

If you feel like you’re always tired in winter, you’re certainly not alone. Unlike the longer, brighter days of summer, the colder months can make it harder to stay active, soak up enough natural daylight, and keep your energy steady.

That dip in sunlight can also take a toll on your vitamin D levels, which play a vital role in keeping your body functioning at its best.

Darker days naturally disrupt your body’s internal clock and boost melatonin production - the hormone that helps regulate your sleep. This can leave you feeling sleepier than usual.

You may also gravitate towards more comforting foods that are high in sugar and unhealthy fats, rather than the nutrient-dense options that fuel your energy.

Combined, these seasonal changes can leave you feeling groggy, and struggling to shake off that persistent winter tiredness.

According to Dr Donald Grant, GP and Senior Clinical Advisor at The Independent Pharmacy, persistent fatigue can sometimes be a sign of an underlying health condition that’s easy to overlook. He advises anyone concerned about their energy levels not to dismiss ongoing tiredness as simply another symptom of winter.

“Feeling a bit more tired as the days get shorter is very common, as less daylight affects our body clock and mood,” says Grant. “But if tiredness is lasting for weeks, isn't improving with rest or is getting in the way of daily life, it's worth considering whether something else could be going on.”

6 causes of winter fatigue that go beyond seasonal tiredness

So, what else could be causing your winter fatigue? Grant highlights some potential causes of tiredness that aren’t simply down to seasonal changes, the signs to look out for, and when it may be time to speak to your doctor.

1. Iron deficiency (anaemia)

Anaemia is a condition in which the body doesn’t have enough healthy red blood cells to carry oxygen effectively around the body. Iron deficiency is a common cause of anaemia and can lead to symptoms that are easily mistaken for winter fatigue - particularly persistent tiredness and low energy.

Grant says that low iron levels are a common cause of ongoing fatigue, particularly among women with heavy periods, pregnant women, and people who eat little or no red meat.

Alongside persistent tiredness, Grant highlights several other symptoms to look out for.

If you’re experiencing these symptoms, it’s worth speaking to your doctor, who can arrange a blood test to check for anaemia and assess your iron levels.

Grant advises against starting iron supplements without medical guidance, as it’s important to establish the underlying cause of your symptoms first.

2. Vitamin D deficiency

Often called the ‘sunshine vitamin’, vitamin D is something our bodies produce when our skin is exposed to sunlight.

While we don’t need sunshine every day to get enough, the shorter days and lack of daylight during winter can make it harder to keep our levels topped up. That’s why adults in the UK are advised to consider taking a daily vitamin D supplement during the autumn and winter months.

Vitamin D plays an important role in keeping our bodies healthy, from maintaining strong bones and teeth to supporting our immune system and normal muscle function.

Low levels can also leave you feeling tired and fatigued, so it’s worth making sure you’re getting enough, especially during the darker seasons.

“From October, the sun in the UK isn't strong enough for our skin to make the vitamin D we need. Low levels can leave some people feeling tired, achy, and generally run down, with bone and muscle pain in more severe cases,” says Grant.

“To maintain a healthy intake, I recommend taking vitamin D supplements throughout the colder months of the year. If you're taking one and still feel exhausted, it's worth looking at other possible causes.”

3. An underactive thyroid

Your thyroid is a small, butterfly-shaped gland in your neck that produces hormones to help regulate several important bodily functions.

As Grant explains, these hormones control how quickly your body uses energy. When your thyroid doesn’t produce enough of them - a condition known as an underactive thyroid - many of your body’s processes can begin to slow down.

This can leave you feeling unusually tired and sluggish. It’s more common in women, and the risk increases with age.

“Tiredness often comes with weight gain, feeling the cold more than usual, constipation, dry skin, and low mood,” he adds. “Many of these symptoms are easy to blame on the colder months, and they can go unnoticed for some time. A blood test can confirm it, and treatment is usually straightforward.”

4. Seasonal affective disorder (SAD)

Säsongsbunden depression (SAD) typically occurs during the winter months, when daylight hours are shorter.

Reduced exposure to sunlight can disrupt your body clock and affect serotonin levels - a brain chemical that helps regulate your mood. These changes can leave you feeling low, tired, and lacking in energy.

Grant explains that SAD is a type of depression that follows a seasonal pattern, usually beginning in autumn and improving in spring.

“Getting outside in daylight, staying active, and keeping in touch with others can all help,” he advises. “If your symptoms are affecting your daily life, speak to your doctor. There are effective treatments available.”

5. Sleep apnoea

Obstruktivt sömnapnésyndrom (OSAS) is when your breathing repeatedly stops and starts while you sleep because your airway becomes temporarily blocked.

It can cause loud snoring, waking up gasping for air, morning headaches, and feeling sleepy during the day.

Grant says that if you’re getting a full night’s sleep but still waking up feeling exhausted, OSAS could be the reason.

“It's more likely if you're overweight or drink alcohol regularly,” he notes. “A partner is often the first to notice, so it's worth asking them.”

6. Perimenopause

Perimenopaus is the natural transition, leading up to menopause, usually starting in your mid-forties and lasting several years.

During this time, hormonal changes can trigger a range of symptoms, from poor sleep and mood swings to hot flushes and brain fog. These can take a real toll on your energy levels, and leave you feeling exhausted.

“For women in their 40s and 50s, changing hormone levels during perimenopause can cause fatigue, often alongside poor sleep, night sweats, irregular periods, low mood, and brain fog,” Grant explains. “Because symptoms can build gradually, many women don't connect them straight away.

“Keeping a note of your symptoms can help you spot patterns. If they're affecting your quality of life, speak to your doctor or pharmacist about the support and treatments available.”

Grant concludes that prioritising rest and getting good-quality sleep can help make tiredness more manageable. Regelbunden träning, eating well, and spending time outdoors can also help boost your energy levels.

However, if your fatigue lasts for more than a few weeks or doesn’t improve with these changes, it’s worth checking in with your doctor.

You should also seek advice if your tiredness is accompanied by other symptoms, such as unexplained weight gain, as there may be an underlying cause.