A Guide to Responsible and Reproducible Vivarium Illumination
Light is a fundamental, yet often overlooked, extrinsic factor in animal-based research. For millennia, life on Earth has evolved under the sun’s rhythmic cycle of light and darkness, which profoundly influences the circadian, physiological, hormonal, metabolic, and behavioral systems of all animals. In a laboratory setting, this natural relationship is frequently disrupted by artificial lighting, which can compromise animal welfare, introduce significant confounding variables, and undermine the reproducibility and transparency of scientific research.
The traditional approach to vivarium lighting has been based on standards originally developed for human visual performance, comfort, and energy efficiency. This has led to the widespread use of metrics like lux, which measure light intensity according to the sensitivity of the human visual system, not the animal’s. As we'll explore, this human-centric approach is insufficient because it fails to account for the complex and species-specific ways that light is perceived and utilized by laboratory animals. A new "animal-centric" paradigm is emerging, providing a more accurate and ethical framework for measuring, standardizing, and implementing vivarium lighting.
The science of animal photoreception
Mammalian eyes contain a diverse array of photoreceptors beyond the traditional rods and cones that mediate image-forming vision. A small but critical subset of retinal ganglion cells, known as intrinsically photosensitive retinal ganglion cells (ipRGCs), contain the photopigment melanopsin and are directly sensitive to light. These ipRGCs form the basis of the nonvisual system, a parallel pathway that regulates a wide range of biological functions, including circadian rhythms, hormone secretion (like melatonin and cortisol), mood, sleep, and pupil constriction.
The key takeaway for vivarium design and management is that the nonvisual system has a different spectral sensitivity than the visual system. While human daytime vision is most sensitive to yellow-green light (around 555 nm), the ipRGCs that drive circadian responses are most sensitive to the blue-appearing portion of the visible spectrum (447 to 484 nm). Consequently, two light sources with the same lux measurement (human-perceived brightness) can have vastly different effects on an animal's nonvisual system, leading to discrepancies in research outcomes and animal health.
Furthermore, the photobiology of animals is species-specific. Many nocturnal rodents, for example, have a high sensitivity to ultraviolet (UV) light, which is outside the human visual range and is not accounted for by standard lux meters. This highlights the inadequacy of using human-centric metrics and underscores the need for a more accurate method of quantification.
The pitfalls of "business as usual" lighting
The current standard of practice in many animal facilities, which relies on outdated guidelines, presents several significant problems.
Circadiandisruption: Light is the most powerful regulator of the circadian clock. Inappropriate light intensity, spectrum, or duration can desynchronize an animal's internal clock, a condition known as chronodisruption. This has been linked to a host of negative health outcomes in both humans and animals, including metabolic diseases, immune system suppression, and enhanced tumor growth. For example, studies have shown that exposing rats and mice to even very low levels of light at night (LAN) can disrupt their circadian rhythms, alter neuroendocrine and neurobehavioral responses, and promote tumor progression. The International Agency for Research on Cancer (IARC) has even classified shift work, a proxy for LAN, as a probable carcinogen.
Compromised reproducibility: The variability of light within a single animal room or across different facilities can be a major confounding variable. Light intensity can vary by as much as 80-fold on a single rack, and by more than 10-fold between the front and back of a single cage. This means that animals in different cages or on different shelves are subjected to different light environments, leading to significant inter-animal variability that can obscure or distort research findings. The lack of an agreed-upon metric for light measurement and reporting makes it difficult to compare research findings or replicate experimental conditions across laboratories.
Ethical implications: Inappropriate lighting can cause undue stress and distress in research animals. The guidelines of the Guide for the Care and Use of Laboratory Animals (the Guide), while a starting point, are considered antiquated as they provide limited guidance on the management of modern lighting technologies, such as Light-Emitting Diodes (LEDs). By not adequately addressing animal photobiology, the current system may fail to ensure the best possible health and well-being of the animals, which is a core tenet of the 3Rs (Replacement, Reduction, and Refinement).
The path forward: recommendations for a new lighting paradigm
A recent expert workshop on circadian and neurophysiological photometry has developed a consensus view on how to address these issues. This new framework moves beyond the traditional lux measurement to a more accurate and meaningful approach for quantifying light as experienced by the animal.
The recommended method for light quantification is alpha-opic irradiance, which measures light based on its effective intensity for each of the four main mammalian retinal photoreceptor types: rods, cones (short- and middle-wavelength sensitive), and melanopsin ipRGCs. This approach provides a minimum of four distinct values that, together, fully describe an animal’s experience of a light source, including its intensity and spectral quality.
For general husbandry, a simpler, single metric—melanopic equivalent daylight illuminance (melanopic EDI)—is proposed. This metric is a good approximation for effective intensity across a wide range of light sources and is more directly related to the circadian system.
For architects, lab planners, and managers, the following recommendations, based on recent research and expert consensus, should be considered:
Embrace a new metric: Move away from lux as the sole measure of light intensity. Instead, use radiometric units (μW/cm2) or the new alpha-opic EDI metric. While specialized meters are still being developed, simple tools exist, such as the rodent irradiance toolbox, that can convert spectral power distribution data into meaningful alpha-opic values.
Standardize and report: Consistently monitor, record, and report light-phase and dark-phase light levels in both the macroenvironment (the room) and the microenvironment (inside the cage, at eye level). This information is crucial for ensuring experimental reproducibility and accountability.
Provide appropriate light levels: For general animal housing, the recommended light-phase intensity is a lower range of 100 to 400 lux (41 to 163 μW/cm2) for most research animals. For rodents, in-cage light intensity should not exceed 75 lux (31 μW/cm2).
Emulate natural cycles: Install lighting systems that can gradually change intensity and spectral quality at the beginning and end of the light phase, mimicking natural dawn and dusk. This can reduce stress and positively influence animal health.
Eliminate light at night (LAN): Strive for a complete absence of light during the dark phase. LAN contamination, which can come from door leaks, observation windows, or lighted equipment, can be easily remedied with blackout curtains, light-tight door seals, and covering indicator lights on equipment. If night work is necessary, limit dim red safety lights to less than 35 lux for no more than 15 minutes.
Consider cage and enrichment factors: Be mindful that cage type, color, and enrichment devices can alter the light experienced by the animal. To minimize variability, standardize these elements and consider using enrichment materials that do not block light.
The evolution of our understanding of animal photobiology, particularly the role of the nonvisual system, demands a fundamental shift in how we approach vivarium lighting. The current practice of relying on human-centric metrics is inadequate and can compromise animal welfare and scientific integrity. By adopting the new alpha-opic irradiance and melanopic EDI metrics, and by implementing practical strategies to standardize light exposure, we can create an environment that better supports the physiological needs of laboratory animals. This not only enhances their well-being but also leads to more robust, reproducible, and ethically sound research outcomes. Moving forward, a collaborative effort among architects, planners, equipment vendors, and researchers is essential to ensure that vivarium lighting is no longer a hidden source of variability, but a carefully controlled variable that benefits both the animals and the science they support.
References
Lucas RJ, Allen AE, Brainard GC, Brown TM, Dauchy RT, Didikoglu A, et al. (2024) Recommendations for measuring and standardizing light for laboratory mammals to improve welfare and reproducibility in animal research. PLoS Biol 22(3): e3002535. https://doi.org/10.1371/journal.pbio.3002535
Dauchy, RT, & Blask, DE (2023). Vivarium lighting as an important extrinsic factor influencing animal-based research. Journal of the American Association for Laboratory Animal Science, 62(1):3-25. https://pubmed.ncbi.nlm.nih.gov/36755210/
Dauchy, RT, et al. (2024). Light: An extrinsic factor influencing animal-based research. Journal of the American Association for Laboratory Animal Science, 63(2):116-147. https://pubmed.ncbi.nlm.nih.gov/38211974/
