Bright side of the night – 2.1.1

Biological rhythms as information for many organisms

Circadian rhythms

Natural light conditions are a crucial prerequisite for the functioning of the biological clock of various living organisms. In particular, small, inconspicuous, gradual changes in natural light intensities and spectra during twilight and night times and over the change of seasons provide the basic information for the circadian—or biological—day/night, lunar and annual rhythms. These biological rhythms have developed over millions of years, and are the basis for the regulation of a multitude of physiological processes, as well as for the timing of life processes and behaviour in general.

In the darkest hours

Half the world has always been dark. The night is as essential as the day. Night is a time of regeneration or a time of activity. When it gets dark, the hormone melatonin is produced in the bodies of all vertebrates. Several important hormones and fixed action patterns may have endogenous rhythms that are entrained by photoperiod. This section introduces information on the effect of photoperiod on endogenous rhythms and daily patterns associated with endocrine function, metabolic rates, activity, and behavior.

Also trees rest at night: By means of laser scans, it was shown that the leaves and branches of the slope birch (Betula pendula) sink continuously downwards in the course of the night hours, reaching their lowest position a few hours before sunrise. In the morning, they return to their original position. These movements can amount to up to ten centimeters. Plant movements are closely related to the water balance. The water balance, in turn depends—among other things—on whether light is available for photosynthesis.

Importance of the moon cycle

Many taxonomic groups exhibit reproductive behavior synchronized to lunar cycles, including fish species, eels, marine polychaetes, and mayflies. To reduce predation, reproduction is often timed to the darkest part of the month—at the new moon. Crayfish shed their skin mainly during dark phases of the moon—also in an effort to protect themselves from predators.

For other animals, the full moon is a central moment. Common toads, for example, arrive at spawning waters at full moon, and more clasping females can be spotted. Eagle owls have been observed, especially when the moon is full, covering longer distances and flying faster. The light of the moon is of central importance for foraging—which in this case also means pollination—of nocturnal and crepuscular bees as well as some diurnal bees. Mayflies also coordinate their life cycles so that they hatch during a bright moon in order to reproduce. Nocturnal mammals, in contrast, avoid open areas under the light of the full moon (max. 0.3 lx) to avoid being seen by potential predators.

Long days and short days

Organisms also react to the change of the length of daylight in the course of the seasons. Once the day length shortens in winter, some insects enter diapause. This is a state of rest during development, when, for instance, moth caterpillars pupate. Also, plants connect their development to the length of the day. The decisive factor for the plant is not the presence but the absence of light. Longer days in spring give the plant the impulse to sprout leaves, shorter days in autumn trigger the influx of nutrients, bud formation and leaf fall. Botanists differentiate between long-day plants and short-day plants.

Toolkit index

Bright side index

Next page

*Lunar Cycle. Image by susan-lu4esm ̶ Pixabay*

Further resources

Links below will redirect you to external websites. In accordance with the European data protection declarations, we would like to point out that by clicking on these links you may send data to external providers. We cannot prevent that.

Videos

Why you should care about light pollution: impacts on nature, humans and animals