Why the winter sun is too low for vitamin D.

Quick answer. Yes, the sun is much lower in winter. At midday on 21 December it reaches about 15 degrees above the horizon in London and about 11 degrees in Edinburgh, against about 62 and 57 degrees on 21 June (calculated). Low sunlight crosses far more atmosphere, and the ozone in it absorbs the UVB that skin needs to make vitamin D, which is why the NHS says "between October and early March we do not make enough vitamin D from sunlight".

Is the sun lower in the winter?

Yes, and by a lot. The Earth's axis is tilted, and as the Royal Observatory Greenwich puts it, "it is tilted 23.4 degrees on its axis" (Royal Museums Greenwich). In December the northern half of the planet leans away from the sun, so the sun climbs less high in the sky and spends fewer hours above the horizon.

The height the sun reaches at midday is easy to work out. We calculated it for UK cities using the NOAA solar position method. In London the midday sun drops from about 62 degrees in June to about 15 degrees in December. Further north the drop is bigger.

What is the sun angle in the UK in winter?

These are the sun's heights at midday on the 21st of each month, in degrees above the horizon. Lerwick in Shetland is included to show how much the north loses.

Calculated for 2026 to 2027, rounded to the nearest degree. That is the highest the sun gets all day. Every other hour of a winter day it is lower still.

One way to picture it: in London the sun at noon on the shortest day is no higher than it is at about 06:40 on a June morning. In Edinburgh the December noon sun matches a June morning at about 06:10.

How long is your shadow in winter?

Your shadow is a quick guide to the sun's height. At 15 degrees, a shadow is about 3.7 times as long as the person casting it. In Edinburgh on the shortest day it is about 5.3 times as long, and in Lerwick nearly 9 times. At midday in June your shadow in London is about half your height. (All calculated from the sun's angle.)

Why can't skin make vitamin D in a UK winter?

Skin makes vitamin D using UVB, a narrow part of sunlight. The Met Office lists the main things that decide how much UV reaches the ground, starting with "the elevation angle (height) of the sun in the sky". It adds that ozone high in the atmosphere "absorbs much of the UV radiation before it reaches ground level" (Met Office).

A low sun makes both worse at once. Its light comes in at a shallow angle, so it passes through far more air and ozone before it reaches you. At 15 degrees, the path through the atmosphere is about 3.8 times as long as when the sun is overhead, and at Edinburgh's 11 degrees it is about 5.3 times (calculated). Much of the UVB is absorbed on the way.

A classic study measured this directly. Researchers exposed skin samples and the vitamin D precursor to sunlight on clear days at different latitudes. In Edmonton, Canada, at 52 degrees north, about the latitude of London and Birmingham, they found that "this ineffective winter period extended from October through March" (Webb, Kline and Holick, 1988). That is the origin of the phrase "vitamin D winter".

The NHS advice follows the same calendar. It says that from about late March or early April to the end of September, "most people should be able to make all the vitamin D they need from sunlight", but "between October and early March we do not make enough vitamin D from sunlight" (NHS).

Does a bright, sunny winter day help?

For vitamin D, no. Clear skies remove the cloud but not the angle. On a cloudless December day the sun is still at 15 degrees in London, and the UVB still has to cross the same long path. NICE notes that "solar UV levels are highest around the middle of the day when the sun is highest in the sky" (NICE NG34), and in December the middle of the day is not high enough. Sitting by a sunny window does not help either: NICE also says the UVB that makes vitamin D "does not penetrate glass".

Winter daylight is still worth getting outside for. It just does not do the vitamin D job. For when the summer sun does, see our guide to how much sunlight you need for vitamin D.

What does the NHS advise instead?

Because the sun cannot cover it, the NHS turns to food and supplements. In its words, during autumn and winter "you need to get vitamin D from your diet because the sun is not strong enough for the body to make vitamin D", and "everyone (including pregnant and breastfeeding women) should consider taking a daily supplement containing 10 micrograms of vitamin D during the autumn and winter." Some people are advised to take it all year. The detail is in our guide to vitamin D in a UK winter.

The low sun also brings dark mornings and early sunsets. If those are the harder part of winter for you, our piece on the clocks going back has a routine that copes.

Where Immufix fits.

Immufix is not a vitamin D supplement and does not replace one. One scoop (25g) in around 200ml of water, once a day, contains 1.25 micrograms of vitamin D3, which is 25% of the nutrient reference value. The NHS advises 10 micrograms a day in autumn and winter, so if you follow that advice, take a dedicated vitamin D supplement and count the 1.25 micrograms in your total.

Vitamin D contributes to the maintenance of normal bones and teeth and to normal muscle function. B vitamins and magnesium contribute to normal energy-yielding metabolism and to the reduction of tiredness and fatigue. Every amount is listed on the science page.

The short version.

• The sun is far lower in winter because the Earth's axis is tilted 23.4 degrees.
• At midday on 21 December it reaches about 15 degrees in London and 11 in Edinburgh, against about 62 and 57 in June (calculated).
• Low sunlight crosses several times more atmosphere, and ozone absorbs the UVB skin needs.
• The NHS says we do not make enough vitamin D from sunlight between October and early March, sunny days included.
• The NHS advises everyone to consider 10 micrograms of vitamin D a day in autumn and winter. Immufix has 1.25 micrograms per scoop.

Food supplements should not be used as a substitute for a varied, balanced diet and a healthy lifestyle. If you have a medical condition, are pregnant, breastfeeding or taking medication, speak to your GP or pharmacist before taking any food supplement.