Light Pollution: Disrupting Natural Rhythms And Photoperiods

Light pollution is a pressing ecological concern that can negatively impact fitness and impair laboratory studies. It refers to artificial light that reduces natural levels of darkness at night, commonly associated with urban areas. The increasing prevalence of exposure to artificial light at night (ALAN) in both field and laboratory settings disrupts photoperiodic time measurement and may block the development of appropriate seasonal adaptations.

Changes to photoperiod (day length) occur in anticipation of seasonal environmental changes, altering physiology and behaviour to maximise fitness. For photoperiod to be useful as a predictive factor of temperature or food availability, day and night must be distinct. ALAN can derange the assessment of photoperiod and impair seasonally appropriate adaptations.

Light pollution can affect the spring development of trees and shrubs, with low-intensity and medium-intensity light pollution treatments resulting in accelerated leaf development in some species. It can also impact conifers, with reports of elongated buds potentially due to light pollution from street lights.

When growing photoperiod cannabis plants, darkness is just as important as light. Light pollution from artificial lights can disrupt the dark period, causing issues such as reversion to the vegetative stage, stunted growth, and even death.

Circadian rhythm disruption

One crucial effect of ALAN is its ability to disrupt melatonin production. Melatonin is a hormone that helps regulate sleep and is typically secreted at night in both nocturnal and diurnal vertebrates. However, exposure to ALAN can suppress melatonin levels, as seen in nocturnal mouse lemurs exposed to moderate light pollution. This disruption can have far-reaching consequences, including altered reproductive functions and immune responses.

For example, in male mouse lemurs, exposure to ALAN resulted in suppressed melatonin levels and the onset of testicular growth and function, indicating a disruption of seasonal breeding patterns. Similar changes in reproduction have been observed in other animals, such as great tits, European blackbirds, and fruit flies.

ALAN can also impact immune function, which is closely linked to survival. Normally, short days in winter are associated with enhanced immune responses, as reproductive costs are minimal. However, ALAN can interfere with this natural rhythm, leading to detrimental effects on immune function. Studies in laboratory rodents have shown that ALAN impairs daily circulation of immune cells and reduces immune responses, making individuals more susceptible to disease.

In addition, ALAN can disrupt metabolic processes. Constant light exposure and high-fat diets have been linked to increased weight gain, altered insulin sensitivity, and disrupted melatonin and corticosterone rhythms. These metabolic disruptions can have long-term consequences on overall health and increase the risk of metabolic disorders.

The effects of ALAN on circadian rhythms and photoperiodic responses are widespread and concerning. From altered reproductive functions to impaired immune systems and disrupted metabolism, the impact of ALAN on circadian rhythms is far-reaching. Addressing light pollution and reducing exposure to ALAN is crucial for maintaining the health and well-being of both human and animal life.

Reproduction

The reproductive system is one of the many physiological systems that are influenced by photoperiod. Photoperiod refers to the period of time in a day that an organism is exposed to light. Photoperiodism is the ability of animals and plants to use day length or night length, resulting in life-historical transformations, including seasonal development, migration, reproduction, and dormancy.

The reproductive system of animals is influenced by the photoperiod. In mammals, melatonin provides the hormonal signal transducing day length. Duration of pineal melatonin is inversely related to day length and its secretion drives enduring changes in many physiological systems, including the HPA, HPG, and brain-gut axes, the autonomic nervous system, and the immune system.

The reproductive system of plants is also influenced by the photoperiod. Photoperiod is an environmental cue linked to several seasonal developmental processes in plants, including the initiation of both reproductive and vegetative organs. The ability to use photoperiod as an environmental cue for regulating a seasonal/annual response is termed photoperiodism.

Immune function

Optimal immune function is critical for survival. Indeed, immune function is often considered a proxy for survival costs in life history approaches. Immune function requires significant energetic investment for ideal function and protection against disease, and these costs must often be balanced against reproductive costs to optimize fitness. Thus, energetic trade-offs have often evolved to balance seasonal challenges, resulting in season-specific investments.

Photoperiodic changes across seasons contribute to survival. The short days (long nights) of winter are often associated with enhanced innate immune responses in field studies; in small mammals and birds, this short-day enhanced investment in immune function occurs when reproductive costs are minimal. This strategy allows investment in survival mechanisms such as immune function and thermoregulation when food resources are most limited. Short day conditions in laboratory-based studies are associated with enhanced immune function when other factors are constant. Presumably, this tactic combats seasonal variance in disease incidence and severity.

The increasing prevalence of exposure to artificial light at night (ALAN) in both field and laboratory settings disrupts photoperiodic time measurement and may block development of appropriate seasonal adaptations. Many studies evaluating the role of ALAN on immune function have used laboratory rodents, and the vast majority indicate that ALAN has detrimental consequences. Dim and ecologically relevant levels of ALAN impaired daily circulations of monocytes and T-cells, along with a reduction in blood monocytes and Cd68. Dim ALAN also impairs cell-mediated immune responses in nocturnal rodents with robust pineal melatonin rhythms, including Siberian hamsters and rats. ALAN also alters immune function in diurnal Nile grass rats. In this study, male Nile grass rats increased bactericidal capacity, pinnal swelling in a cell-mediated delayed-type hypersensitivity test, and upregulated antibody production post keyhole lymphocyte hemocyanin. Notably, studies performed in nocturnal mice without robust pineal melatonin rhythms have demonstrated that exposure to ALAN induces increased proinflammatory cytokines and enhanced inflammatory response to LPS. Exposure to ALAN resulted in a greater pinnal swelling in a delayed type hypersensitivity reaction in short day hamsters relative to long day animals. Other results of this study indicated that ALAN interfered with short day phenotype development which can negatively impair photoperiodic reproductive responses, survival, and fitness.

Metabolism

Light pollution, or artificial light at night (ALAN), can have a significant impact on the metabolism of various organisms, including humans. Here are some key points on how light pollution affects metabolism:

• Circadian Rhythm Disruption: Light pollution can disrupt the body's internal circadian rhythm, which is crucial for regulating metabolism. This disruption can lead to changes in hormone signaling, insulin resistance, and increased risk of metabolic disorders such as obesity and diabetes.
• Melatonin Suppression: Light pollution suppresses the production of melatonin, a key hormone that regulates sleep and circadian rhythms. Melatonin also plays a role in metabolism, and its suppression can have downstream effects on energy balance and glucose metabolism.
• Altered Feeding Behavior: Light pollution can impact feeding behavior, leading to increased food intake and potential weight gain. This may be due to the disruption of the body's natural light-dark cycles, which normally regulate feeding patterns.
• Immune Function: Light pollution can negatively impact immune function, which is closely linked to metabolism and overall health. A well-functioning immune system requires significant energy investment, and light pollution can disrupt the body's ability to balance immune function with other metabolic demands.
• Thermoregulation: Light pollution can affect thermoregulation, which is the body's ability to maintain core body temperature. This, in turn, can impact energy expenditure and metabolism.
• Plant Metabolism: Light pollution can also affect plants, altering their natural photoperiod and disrupting their metabolism. This can have consequences on their growth and development, particularly in urban areas with high levels of light pollution.

Thermoregulation

ALAN disrupts the measurement of photoperiod (day length) and may block the development of appropriate seasonal adaptations, including thermoregulation. This is because the clear differentiation between day and night is critical for the assessment of photoperiod, and ALAN can disrupt this by altering melatonin secretion, which is influenced by the duration of night-time.

ALAN has been shown to impair thermogenesis in social voles, reducing their ability to regulate body temperature. Similar effects have been observed in other species, such as kangaroo rats and Indian weaver birds, where ALAN exposure reduced core body temperature and energy expenditure.

The impact of ALAN on thermoregulation may be due to its effect on the suprachiasmatic nuclei (SCN) in the brain, which plays a crucial role in coordinating time-of-day cues. Alternatively, ALAN may alter thermogenesis through the disruption of thyroid hormone secretion, as thyroid hormones play a key role in regulating body temperature.

Overall, ALAN has been shown to disrupt thermoregulation in animals, potentially impacting their survival and fitness.

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