How do we count the days? By bracketing them with night. We rise with the dawn and remain active into the evening, confident that when awakened by sunlight, the “next” day will be distinct from those before.
But there is no break in time’s continuum, no division between the days.
Night’s blanket shades half the spinning globe and creates twin hemispheres of light and dark. For us, the world of darkness is filled with unfamiliar lives flourishing under unfamiliar conditions, creatures and blooms for which daylight is too competitive or hazardous.
Flowers of the night fold up in the sun’s glare as a host of animals hide or sleep in preparation for hunting and grazing in the forthcoming darkness. Night is the time of cricket and whippoorwill songs, the flight of bats and owls, the time for pale blossoms to open and waft heady scent to draw night-flying pollinators.
Pigmentation of plants and animals is affected by diurnal activity. Night-blooming flowers are conservatively white, sometimes huge, and sought by nocturnal moths. Multihued daytime flowers are sought by bright hummingbirds, butterflies, bees, and iridescent flies.
Because plants require light for their photosynthetic processes – which in turn nourish animals that “burn by the power of the leaf” – our green neighbors respond to alternating periods of light and dark. Wide, light-catching leaves droop after sunset as dandelions close. Plants flower, night or day, in relationship to the stimulus of darkness that adjusts reproductive readiness to be triggered by daylight.
Small crustaceans, jellyfish, and pelagic worms, activated at regular intervals by light filtering through the surface waters of lake or ocean, migrate vertically in a 24-hour cycle, rising at night, descending in daylight.
Land-dwelling millipedes, slugs, and bats also show clearly defined day and night rhythms. Scarab beetle larvae emerge from their burrows after dark to feed when they are least subject to attack by predators.
The subtle lengthening of days stimulates newly arrived birds to develop brighter colors and leads to courtship rituals and springtime breeding. When days shorten a few months later, colors pale and a long southward migration is undertaken by many species according to each bird’s internal clock. Some navigate by starlight.
Animals active by day rely heavily upon sight to find food and avoid enemies, but nighttime animals depend more upon scent, touch, and hearing. If eyes are used at all, those of nocturnal creatures tend to be large and especially sensitive to dim light, but not to color. Although camera-like eyes are marvelous organs for light detection, they are not the only plan. Many microorganisms respond to light, and even the eyeless earthworm distinguishes the safety of night from risky day by light-sensitive cells at the front of its slender body. It emerges to forage in the dark while robins sleep.
The great searching eyes of nighttime bird and mammalian predators are sensitive to a degree we will never know, packed as they are with retinal cells that distinguish infinitesimal movement and variations in gray, but never color. Nocturnal animals themselves tend to be dull in color, or conspicuous in warning black and white. Not even the skunk wants to be stumbled over in the dark.
The deer mouse runs and feeds all night, singing and cleaning its nest before a day’s sleep. Other creatures of the dark diminish competition and avoid nocturnal predators by being segregated from one another within specific periods of nighttime activity. Some are active as soon as the sun sets, some not until two hours later, some at midnight, and some around 2:00 A.M. Each kind of mosquito has its scheduled time to bite, and cockroaches awaken one or two hours after dark to scuttle about, retiring in plenty of time to hide before the advent of dangerous day.
Living jewels of sunlight include scarlet dragonflies, emerald tiger beetles, and indigo buntings, while green katydids and brown moths take on the hues of leaves and bark. Some daytime caterpillars display markings that resemble a snake’s head, while others rear up from a branch to imitate a twig. Threatened by a nocturnal predator, certain nighttime moths uncover their under-wings to reveal the staring “eyes” of an owl outlined by tiny scales. In dim light, others at rest resemble bird droppings. If kept in the dark, one small shrimp changes color every twelve hours, keeping time with the last exposure it had to daylight.
The twins of daylight and shadowed night govern inhabitants of land and shallow sea. Communities of living things sleep and waken to the poet’s “dance of life,” for it is the poets who sing of sunbeams, of night and moon and stars, of dreams and romance, of courage and fear, revealing our own rhythmic dancing to the cosmic tune.