This is the second part of a 2-part series of blog posts exploring the idea of nighttime as a habitat. You can read Part 1 here.
In Part 1, we mentioned that the same habitat can look very different at night due to the difference in light levels and being inhabited by different creatures. In this post we want to expand on this and introduce the idea of night as a habitat situated in time as well as in space, a temporal habitat. Living creatures need both space and time in the conditions they have adapted to in order to survive.
The biological processes of all living beings can be split into two broad categories: activity and rest. Activity encompasses the things creatures do to survive through gathering energy and reproducing. Rest is the other portion — conserving energy for physical restoration and repair. The most explicit form of rest is sleep, which many animals do, but creatures who don’t appear to sleep like spiders, insects, and bullfrogs, also lower their metabolic rates to rest with their eyes open. Even microorganisms show patterns of activity and rest.
Most patterns of rest in living creatures have circadian rhythms, regulated by an internal biological ‘circadian clocks’ that are calibrated by cycles of day and night. Diurnal animals rest when it’s dark and stay awake during the day, and nocturnal animals do the opposite, resting when it’s bright and resuming activity in the dark. Crepuscular animals live on the boundaries of these light changes, being most active at dusk and dawn when the light is changing, and resting outside these windows.
In this way, daylight, night, and twilight are windows of time in which different living creatures can be active. The same physical spaces are inhabited by different groups of creatures depending on where you look (in an illuminated space vs. a dark space), but also depending on when you look (during the day, at twilight, or at night). These three broad levels of light provide different environmental conditions that allow these creatures to base their activity on, and these conditions are radically altered by the presence of artificial light. Artificial night light, especially the broad spectrum LED fixtures abundant on roads and buildings, mimics high intensity daylight in its composition and in the fact that it usually shines from above.
Every place experiences an equal balance of daytime and nighttime. The specific amounts of light and dark can vary from day to day for instance, in parts of the world that experience seasons where some nights are short and others are long, or in a more extreme example at the Earth’s poles, which experience 6 consecutive months of dark and light alternately. But over the course of a year, the amount of light and dark averages out to half and half. Human lighting that mimics daytime extends the conditions of daytime into the window reserved for night, and in many cases, erases twilight conditions altogether.
Many areas have sensor based lights that switch on right when the intensity of daylight dips usually around sunset. These lights typically shine throughout the night and switch off when natural light intensity roughly matches daylight (at dawn). In Part 1, we propose that the same physical area serves as different habitats depending on whether there is light shining on it or not. We want to extend this idea to include the time dimension — day and night, darkness and light, are cycles in time, and when an area is illuminated through the evening and night, the habitats of crepuscular and nocturnal animals are being degraded. If the spaces these animals occupy are illuminated by what seems like daylight for 24 hours a day, there is no area for crepuscular or nocturnal animals to occupy, and as naturally dark areas become less and less available, more of them are forced into smaller areas by anthropogenic light, and their natural habitat gets smaller.
Nocturnal (and crepuscular) habitats are necessary for diurnal creatures too — darkness is a physiological signal that cues our physiology to rest and replenish. Light that mimics daylight, whether from screens or outdoor lights, prevents this rest with powerful biological stimulation, leading to poor rest in the short term and many health complications in the long run.
By keeping areas as naturally dark as possible, you are reversing a silent and innocuous form of habitat degradation that happens much more subtly — habitat erosion in time as well as in space. Restoring natural darkness by keeping artificial lighting to a bare minimum doesn’t only give habitats back to crepuscular and nocturnal creatures — it also helps diurnal creatures rest and rejuvenate properly.