One of the big mysteries of biology is precisely how living things, including us, are affected by the seasons. We know we are—we feel it most acutely in the darkest days of winter—but how does it work?
There are a few clues and there's a lot to observe. This year's winter solstice—when the sun is farthest from the equatorial plane—falls on Dec. 21 at 7:22 p.m. ET. So, we're approaching the midnight of the year—the time of minimum sunlight and maximum night—when most of us feel a little dormant, a bit groundhogish. Some animals and birds have moved south, some are curled up in a cozy den. Some people have also flown south.
Those of us who are staying put experience a semihibernation. (Until March, I pretty much feel as though I've just gotten off an airplane flight from Moscow.) We are sluggish, gloomier than usual, sleepier; we're eating pasta with three kinds of bread; and we're tempted in our low state to drink terrible things like eggnog and pumpkin-spice latte.
The green things of the world, which don't have the option of migrating or overeating, are affected more drastically. Most trees and bushes are bare, annual plants have gone to seed and died, perennial plants have died back to their roots. (Note that we're talking temperate zone, Northern Hemisphere, and not counting evergreens.)
Because they can't migrate or dress warmly, plants are more alert than we are about planning for winter. Back in midsummer, while we were still gaily drinking gin and tonics and wearing flip-flops, plants began to prepare to ride out their most difficult season. By the end of summer, long before temperatures began to fall, they had noted shorter day lengths and set their winter buds. They could go dormant having provided for the season after winter, the season when daylight would increase, signaling leaves and flowers to unfurl.
Plants respond to shorter days faster than we do because it's a matter of life and death for them. A plant's life is all about light, the force they use to produce their food. Plants absorb the energy of light, using it as the motor to power a rearrangement of molecules of water and carbon dioxide to make oxygen and sugar, giving the world its food and most of its oxygen.
How do plants sense that the days are getting shorter? They have, in a sense, both a clock and a calendar. It's a chemical clock, and it works at the level of the cell. Just like most mammals, plants have circadian rhythms, fluctuations of proteins that occur over the length of the day. A short day leaves some of the daytime functions unfinished. This is a hot topic for plant biologists, especially because global warming may cause plants to die out in their normal range and begin to colonize areas farther north. Dogwood trees, for example, that used to freeze north of New York will begin to grow in, say, New Hampshire. Given a warmer world, the temperature may be right, but the winter days will be shorter and the angle of light darned confusing.
A plant's calendar tells it the difference between a short day in November and a short day in March. In early spring, plants have accumulated a cellular memory of winter going by; they know warm days are coming and it's safe to leaf out.
Like plants, we have an inner clock that tracks the seasons. Luckily, here in the dead of winter, we're not experiencing something similar to losing all our leaves, but we do note, at a cellular as well as a psychological level, that there are fewer hours of sunlight.
We, too, have ways of monitoring light and we, too, have light-dependent chemical reactions. Light coming into the eyes is registered by a bundle of neurons about the size of a grain of rice. It's called, should anyone ask you, the suprachiasmatic nucleus. Light causes that bundle of neurons to fire, which suppresses the production of melatonin, known as the sleep hormone. Most of us first heard about melatonin when friends tried buying it at health-food stores to reduce jet lag or help in getting to sleep.
Something is awry with the melatonin levels in people who get disabling seasonal depressions—called seasonal affective disorder. Taking a sort of sun bath with a light box for an hour early in the morning seems to work for the severely depressed by fooling the body into sensing that the sun has come up earlier. Hippocrates advised the saddened to look at the sun. We can wonder if there were a lot of eye-damaged people stumbling around Greece in fourth-century B.C.
We don't have the plant's talent for making sugar and oxygen out of sunlight, but we do have a useful chemical reaction using the sun's energy—the way we make vitamin D. The sun's ultraviolet rays hit our skin and shake up molecules, making a form of cholesterol that's on its way to being vitamin D. In the Rube Goldberg way the human body often works, the substance has to be modified first in the liver and then in the kidneys. Vitamin D's main function is to maintain normal blood levels of calcium and phosphorus. You can also consume it in some foods—notably milk fortified with vitamin D, salmon, and sardines.
Recent studies found that from November through February, the average amount of sunlight at about the latitude of Boston and points north wasn't enough to produce significant vitamin D synthesis. Not only people who live in northern latitudes are at risk—those who work in occupations that prevent exposure to sunlight or women who cover their body for religious reasons, for instance. Is there some poor woman dressed in full hijab, working as a nighttime security guard, to whom we should send vitamin D supplements?
We provide consolations to get through winter—vitamin D supplements, smoked salmon, candles, Christmas lights, cassoulet, oatmeal, velvet, Irish whiskey with hot water and lemon. But for all the comforts we give ourselves, for all the heating and bright lighting, we're still basically creatures who have more in common with the furry beings of the world, and even the leafy beings of the world, than we think. Our bodies are inextricably linked to seasonal cycles. I wonder if there was a time in human history when sluggishness in winter was good for survival? Should we be using this semihibernation as a time of renewal? Should we be, figuratively, setting the buds for spring leaves and flowers?