Today is the shortest day of the year - here are 3 surprising ways it's likely affecting your health and mood

Today is the shortest day of the year in the Northern hemisphere. The good news though, is that from now on we're going to get a bit more daylight every day. (Phew!)

The Winter Solstace lasts 7 hours, 49 minutes, and 41 seconds in the UK, which is nearly 9 hours less than the longest day in June.

It happens when the Earth's tilt means that the North Pole is the furthest away from the Sun, giving us the least amount of daylight possible.

So although the winter days are slowly going to get longer, it will still be a while until you actually notice.

Less daylight can have some surprising impacts on our health

A lack of sunlight can have several impacts on our health, both physically and mentally. Our body clocks - called circadian rhythms - rely on the cyclical nature of the day.

We also get hungry and sleepy at particular times, thanks to the scheduling of when different hormones are produced. Disrupting your routine, even for an hour or so, can leave you feeling out-of-whack, a bit like jet-lag.

When the clocks go back in winter, we are suddenly exposed to shorter, darker days that just seem to get longer and darker. This can affect us more than we think. For example, more melatonin is produced when it's dark because it induces sleep, and when it's bright, we produce less. This could be an explanation for why we feel more lethargic when it gets dark earlier.

In July, Public Health England issued new advice, saying that everyone should be getting 10 micrograms of vitamin D daily, either from supplements or food. Vitamin D is made in the skin when its exposed to sunlight, so its no surprise that we might not be getting enough of it now that our daylight hours are shorter.

Vitamin D deficiency

One rather severe potential effect of not getting enough vitamin D is a condition called rickets, or osteomalacia in adults. It's characterised by weak bones and deformities like bowed legs and a curved spine.

The disease was mostly eradicated in the UK and US during the 20th century, largely because breakfast cereal companies started including vitamin D in their recipes. However, in the past few years, rickets has made a bit of a comeback.

In 2013, Chief Medical Officer in the UK Dame Sally Davies said she was "ashamed" that the number of children with rickets was rising. The Department of Health released a report called "Our Children Deserve Better: Prevention Pays," which highlighted the increasing number of children suffering from vitamin D deficiency.

According to PHE, 23% of adults between 19 and 64 and 22% of children aged 11 to 18 have low levels of vitamin D in their blood. That's nearly a quarter and a fifth of people being at risk of developing a deficiency.

As for why this could be, nobody really knows. There are some theories that it could be related to the rise in indoor office jobs, more air pollution blocking out sunlight, children playing outside less, and an ageing, increasingly housebound population.

It's also quite difficult to get enough vitamin D in your diet. It's found in fatty fish like salmon and tuna and there are small amounts in beef liver, cheese, and egg yolks.

You may exercise less

When it's dark and cold, you might feel pretty unmotivated to get out and work out. One study from the London School of Hygiene and Tropical Medicine measured associations between the time of sunset and physical activity levels of 23,000 children, and found that children's total daily activity levels were 15-20% higher on summer days with sunset after 9pm, compared to winter days with sunset before 5pm.

Regular exercise has been shown to play a key role in several areas of health, including helping to protect against the risk of heart disease, obesity and diabetes, according to the Department of Health.

You can also suffer from SAD

Many people claim to suffer from lower moods in winter. For some people, this may be related to a condition called Seasonal Affective Disorder. It was first suggested as a condition in 1980s, and is defined by symptoms such as lethargy, low mood, excessive tiredness, and weight gain. A drop in serotonin levels may play a part in the development of SAD.

There is a treatment for SAD called light therapy to try and combat the condition, but the evidence that it offers substantive benefits for people is patchy. A Cochrane review last year came to the conclusion that the evidence of light therapy being a preventative for SAD was "limited."

So it might not all be in your head if you're feeling low during the dark, winter days. To combat this, try and get out of the office as much as you can, and exercise - even if you don't feel like it.