Ah, winter, the season of long nights.
Human activities naturally change to accommodate the fewer daylight hours, leading to more sleeping and reading, perhaps, and less outdoor work and recreation in the winter. Other fully diurnal (daytime active) animals may have their food-collecting time curtailed in winter, adding to their challenge to survive the season.
Biologically, the relative length of day/night signals living things to do certain activities. Such patterns, called “circadian rhythm,” apply at the cellular level as well as to overall behavior; an organism’s “sleep time,” usually at night, is the time for rest and repair on all levels. Even many microbes have circadian rhythm.
Winter is rather like the night of the year — not just because it’s darker, but also because it’s the quiet time of rest.
So what happens when artificial lights change the game?
Winter or not, people get less sleep when artificial lights are available, working or playing when it’s naturally too dark to do those things. While that might seem a benefit (especially in winter), some research has linked the reduction of sleep to a myriad of health-related problems. According to some recent interpretations of human history, we’re supposed to sleep all night, perhaps with a short wakeful period somewhere in the middle.
Artificial lights can mislead animals regarding migration timing and pathways: if shorter days mean it’s time to move south, for example, too much light could suggest a delayed winter, mis-cueing the migrant to start too late. (A somewhat separate issue from seasonality, artificial lights can misdirect animals — such as drawing baby sea turtles away from the sea or enticing moths away from their natural mating areas.)
Plants use light for seasonal information in addition to using it for making food. If you’ve ever tried to force a poinsettia to bloom by sticking it in a dark spot, you know that day-length/night-length is an important signal for many plants to start flowering. One of the arising challenges of climate change is that in certain temperate zones some of the pollinating insects that are awakened in the spring by temperature start buzzing about before the plants are fully blooming, as the plants are awakened later by the unchanged day/night-length that for them signals the end of winter.
Artificial light in human habitations can trick the plants into changing their behavior by confusing them about the length of day/night, effectively making them “think” winter starts later and ends sooner than it actually does. For example, artificial lights can cause plants to leaf out or flower early in the spring, or to keep its leaves too long in the autumn. Even a bright street light on one side of a deciduous tree can cause the leaves on that side to stay on the branches longer than on the dark side of the tree, effectively delaying the tree’s experience of winter.
Active, rest, active, rest… Both night and winter are the resting times.
Have our pervasive artificial lights improved our experience of nights and winter or degraded it?
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