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.