Everyone in the UK is advised to take a supplement of vitamin D every day to support general health and in particular for bone and muscle health.
Many of us have been indoors more than usual this year and so might not have been making enough vitamin D from sunlight.
Vitamin D helps regulate the amount of calcium and phosphate in the body to keep bones, teeth and muscles healthy.
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A lack of vitamin D can lead to bone deformities such as rickets in children, and bone pain caused by a condition called osteomalacia in adults.
Government advice is that everyone should consider taking a daily vitamin D supplement during the autumn and winter.
People at high risk of not getting enough vitamin D, all children aged one to four, and all babies (unless they're having more than 500ml of infant formula a day) should take a daily supplement throughout the year.
According to the NHS, from about late March/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.
The Department of Health and Social Care recommends that adults and children over four take a daily supplement containing 10 micrograms of vitamin D throughout the year.
But one question many people ask is 'when is the best time of day to take Vitamin D?'
Research has linked vitamin D levels to sleep quality.
Some reports have claimed that taking vitamin D at night can impact sleep quality by interfering with melatonin production.
But the results are inconclusive.
Most people find the morning the best time to take their vitamins and tablets - but this is more for convenience.
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