Spring has two official start dates, depending on your priorities. For meteorologists, spring already sprung on 1 March, 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 in Africa, where our species began life, 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.”
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,” Met Office meteorologist Aidan McGivern 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 2016 trial 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.” The researchers had previously had similar results when the two treatments were administered to people with seasonal major depression.“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 research has associated deficiency in vitamin D with the onset of depression, 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.”
