For people living in the northern hemisphere, spring has two official start dates.
Meteorologists consider 1 March a spring day, according to the seasons formalised in the 1900s. However, the astronomical seasons show spring starting at the vernal equinox, which falls on 20 March.

The two spring dates

The spring and autumn equinoxes lie halfway between the shortest and longest days of the year. At these points, day and night are mostly even lengths all over the world.
These conditions may suit the human circadian rhythm - the daily cycle that affects the body's sleep, wake, eat, and other biological processes. The benefits of a well-entrained body clock include modifying your metabolism, decreasing cardiovascular disease and better eyesight.

Equinoxes and the circadian rhythm

The good light at the onset of spring is brighter and makes spring feel so central. Daylight is noticeably increasing in length, and the sky is bluer.
Bright light can make us happier. It can be as effective in treating depression as Prozac. Some data reveals that light exposure during the day is related to the quality of sleep, which will affect emotions and systemic health.

Effects of bright light

Research associates a deficiency in vitamin D with depression. Spring is when we can expect to start making vitamin D from sun exposure. 
However, if the UV index is less than two, then you will not make enough vitamin D in a reasonable time. To get the full benefit, you have to expose unprotected skin.

Spring and Vitamin D

Warmer days and nature awakening are not the only reasons to be cheerful. Scientists explain exactly why this season makes us feel better

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Warmer days and nature awakening are not the only reasons to be cheerful. Scientists explain exactly why this season makes us feel better

Last modified on Wed 17 Mar 2021 09.05 EDT

L ike prisoners waiting to be released from winter, we on these small islands in the northern hemisphere have been willing spring on for weeks now, watching for signs. The hours of light are growing apace, as the shadows shrink. Chilly daffodils nod at us from municipal flower beds. Are the skies even getting bluer? Our senses are alert in ways we don’t fully understand, like a pleasing, hazy inheritance from the wild creatures we once were.

Spring has two official start dates, depending on your priorities. <a data-link-name="in body link" dir="ltr" href="https://www.metoffice.gov.uk/weather/learn-about/weather/seasons/spring/when-does-spring-start">For meteorologists, spring already sprung on 1 March </a>, according to their neat, evenly spaced seasons, formalised in the 1900s. But if you plot the seasons in line with our planetary activity, as humans have done for thousands of years, the “astronomical” seasons show spring starting at the vernal equinox, which this year falls this year on 20 March. Just a few days to go …

The equinoxes (spring and autumn) lie halfway between the shortest and longest days of the year. At these points, fleetingly, day and night are of roughly even lengths all over the planet – closer to conditions <a data-link-name="in body link" dir="ltr" href="https://www.theguardian.com/world/2020/jan/05/the-search-for-eden-in-pursuit-of-humanitys-origins">in Africa, where our species began life </a>, and where seasonal swings in daylight hours are less dramatic, especially closer to the equator.

These conditions may well best suit the human circadian rhythm – the daily cycle that tells the body when to sleep, wake, eat and carry out various other biological processes. Stuart Peirson, professor of circadian neuroscience at Oxford University, says: “We do all our laboratory experiments in 12 hours of light and 12 hours of dark. It’s a very balanced, neutral middle ground. We know that longer nights and longer days can influence the circadian system.”

The key thing with circadian rhythms, he says, is that even if you live in a cave, your body “will still operate on a 24-hour cycle because we have an inbuilt biological clock that can tell the time”. The clock runs slightly longer than 24 hours, varying slightly from person to person, so the light in our environment adjusts the clock to the correct environmental time, to stop it drifting. “That’s what we call entrainment,” he says. If we’re well entrained, we’re more likely to sleep well at night, and feel good when the sun’s up.

Good light is why the onset of spring feels so pivotal - it certainly isn’t the temperature. “You’re more likely to get snow in March than you are in December,” says the Met Office meteorologist Aidan McGivern. “It’s just a lot brighter.” There is a sudden jump in the hours of sunshine we get, which is dictated by both daylight hours and weather conditions. “December is the dullest month, with an average 41 hours of sunlight in the UK,” says McGivern. “In January, it’s 47 hours, February jumps to 70 hours and then there’s quite a leap into March where we get 102 hours of sunshine. April sees another big leap to 148 hours on average.” Who doesn’t have a cherished memory of sunbathing weather at Easter?

Surprisingly, perhaps, May is the sunniest month of the year, even though the days are longest in June. Overall, says McGivern, “spring is the most settled time of the year. In June you get what is known as ‘a return of the westerlies’. Atlantic weather fronts make a return after being blocked through the spring.” In other words, areas of low pressure and rain-bearing fronts arrive from the Atlantic.

It is no illusion that the lengthening of the days speeds up as we approach spring. The wall opposite my kitchen window is in shadow over winter; in March, a stripe of sunlight appears at the top and within weeks the entire wall is golden. “Days get longer at a quicker rate at this time of year. And that rate of change peaks during the spring equinox,” says McGivern. “And then it slows down around the solstices, like an S-shaped curve. At the moment, daylight is increasing in length by just over four minutes a day, and that is quite noticeable. After seeing so much gloom for a few months, it suddenly becomes brighter much more quickly.”

It’s also no illusion that the sky glows bluer as we leave winter behind. To understand this, you need to be clear about why the sky looks blue in the first place, so here is McGivern’s handy recap: “The sunlight that comes into our atmosphere is made up of all the different colours of the spectrum, but the colours have different wavelengths.” Red light has longer wavelengths, while blue and violet light have the shortest wavelengths. “The oxygen, nitrogen and other molecules that the atmosphere is made up of scatter the light as it bounces off them, and the shorter the wavelength of light, the more efficiently it is scattered. And so you get more scattering of blue and violet light than you do of red light.” The colours with longer wavelengths don’t linger in the sky, but land on the Earth’s surface instead.

So the sky is glowing with beautiful blue light, but when the sun sets, says McGivern, “then it’s closer to the horizon. The light has to go through a much larger slice of sky and so all the blue light gets scattered out before it reaches us.” At this time of year, when the sun climbs higher in the sky during the daytime, it’s going through a shorter slice of atmosphere than it did on winter days. “So the sky does end up being bluer than in the winter,” he says. “It’s also why you get bluer skies when you’re on top of a mountain compared to down in a valley, because on top the sunlight is going through a smaller slice of atmosphere, and also there are fewer pollutants up there compared to closer to the ground.”

The lack of pollutants in 2020’s spring lockdown made for exceptionally blue spring skies. This year, he says, the effect, “probably won’t be to the same extent, but you’ve still got fewer planes in the sky and possibly less road traffic. So compared with a pre-lockdown spring, it may still be bluer.”

As well as the cheering colour of the sky, bright spring light can make us happier. In fact, light can be as effective in treating depression as Prozac. While Peirson says it’s hard to find good data on seasonal affective disorder – because many study participants aren’t clinically diagnosed, anecdotally speaking, it is not uncommon for people to find the dark of winter oppressive to some extent. Some data, he says, “shows that light exposure during the day is related to quality of subsequent sleep.” Which in turn affects mood and systemic health. But there may be more to it. A <a data-link-name="in body link" dir="ltr" href="https://jamanetwork.com/journals/jamapsychiatry/fullarticle/2470681">2016 trial </a>by researchers from a number of institutions in Canada, including the University of British Columbia in Vancouver, treated people with major depressive disorder without a seasonal (winter) pattern, “to compare fluoxetine, which is Prozac, versus light therapy. They found that light therapy was just as effective as fluoxetine in treating depression.” <a data-link-name="in body link" dir="ltr" href="https://ajp.psychiatryonline.org/doi/10.1176/ajp.2006.163.5.805">The researchers had previously had similar results when the two treatments were administered to people with seasonal major depression </a>.“That’s one of the nicest studies that shows mechanistic evidence under clinical trial settings of the role of light in regulating things like emotionality,” says Peirson.

This means it can only be good news that, as well as the days getting longer, light intensity in spring is stronger and continues to improve as we hurtle towards summer. Outside, in winter, says Peirson, the brightness “will be several thousand lux rising to up to 68,000 lux in summer.” Indoors, however, it would be unusual to get more than 500 lux, “depending on windows in the room and how well lit it is,” he says. It is 8.30am and he takes a quick light reading in his living room and finds it is only 100 lux, then steps outside, where there is light cloud covering, into 10,000 lux.

“It makes me sound like your grandma saying: ‘Go out and get some daylight.’ But, actually, daylight is far brighter, and our bodies have evolved to be expecting a bright light cue.” Part of the cumulative effect of spring is that, as the weather gets warmer, we are tempted to spend more time outside.

Over winter, says Peirson, “we may have less well entrained circadian clocks”, but the benefits of a healthy, well entrained body clock are myriad, impacting metabolism, weight gain, cardiovascular disease and even eyesight. “Low light levels during development can lead to myopia (shortsightedness). In countries where they have had high rates of myopia, encouraging children to be outdoors more has had effects on the incidence of myopia. So, as well as the effects of light on things like depression and mood, there are certainly effects on other aspects of our physiology as well.” Alertness is one. If you’re sleeping well because you have had enough light exposure during the day, you will feel more awake and able to concentrate when you need to. Light itself “has an alerting effect”, too, says Peirson.

Emerging <a data-link-name="in body link" dir="ltr" href="https://pubmed.ncbi.nlm.nih.gov/23377209/">research has associated deficiency in vitamin D with the onset of depression </a>, with research continuing into its effects on mood. A boon for keen gardeners – already busy outside doing their seasonal jobs – is that spring is when we can realistically expect to start making vitamin D from sun exposure. “Yes, right about now,” says Ann Webb, professor of atmospheric radiation at Manchester University, “though the exact time varies a bit with latitude – slightly earlier on the south coast than in northern Scotland.” As always in the UK, the effect is weather dependent – it’s better on a sunny day than a dark and rainy one. “As a very rough guide, if the UV index is less than two, then you will not make any appreciable amounts of vitamin D in a practical time period. You do have to expose unprotected skin,” she points out, “so there is limited synthesis when it is cold because people wrap up well and do not expose their skin.”

You do not have to be a gardener to know, however, that nature is a well-established mood-enhancer, and the slow drip of the world’s return to Technicolor feels as important as the light. Peirson says the evidence for human seasonal physiology – things such as hibernation or other physical seasonal changes – “isn’t very strong”. But, historically, humans who have lived and farmed in seasonal climates have been in thrall to spring’s great rebirth. “Any culture that depends on harvesting seasonal food from roots and bushes, or hunting animals such as deer, must be keenly aware of the time of year,” says the archaeologist and TV presenter Francis Pryor. “There’s lots of evidence for the importance of the sun in ceremonial and funerary sites of early farmers. Iron-age roundhouses are laid out so that the doorway faces towards the sun.”

This awareness is vanishing from the modern world, he says, “where people don’t even understand when certain fruit and vegetables are ‘in season’, because so many are flown in from far away”. And yet a deep appreciation of spring persists – how could it not? – drawing us out of our buildings, blinking into the light.

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Let there be light! Why sunny spring days make us happier and healthier

The two spring dates

Equinoxes and the circadian rhythm

Effects of bright light

Spring and Vitamin D

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