What Light Levels are Best for My Rodent Housing Rooms?

This may not be the right question to ask any longer. As we continue to get more sophisticated in our understanding of mouse welfare and ways experiments are affected by non-experimental variables, it is becoming more apparent that we should be asking: What are optimal light levels for laboratory rodents? It’s important to understand that room light levels and mouse exposure levels are two different values.

When setting light levels for laboratory mice, most of us refer to the “Guide for the Care and Use of Laboratory Animals,” which states that rats and mice generally prefer cages with low light intensity and that room light levels of about 325 lux (30 foot-candles) approximately 1 meter (3.3 feet) above the floor is an appropriate guideline¹. But how does room light level relate to light intensity at the cage level? How much light is an individual mouse exposed to in the cage? Can the mouse escape from the light if it chooses to?

Factors that affect light intensity at cage level

Light intensity varies with distance; hence cage-level illumination can change as a function of rack style, the rack’s position relative to the light source, and the cage location within the rack. So, at 325 lux at 1 meter above cage floor, the lux level a mouse experiences within its cage is dependent on where the rack is located within the room, how high or low the cage is on the rack, if the top rows of cages are shielded from the ceiling lights, and whether it is housed in a single-sided or double-sided library-style rack versus a carousel rack. The higher a cage is on the rack and the farther it is from the wall (the closer the cage is to the light source), the higher the light intensity the mouse will encounter.

Why should we care about light intensity?

Light intensity and light cycles can affect the physiology, development, and behavior of rodents and should be controlled in scientific studies. Light cycles are important for maintaining circadian rhythm and in most facilities are set at a 12:12 or 14:10 for the light:dark cycle. We often keep light in housing rooms at levels for staff to properly see to perform their daily tasks, but rodents prefer low light intensity² and for most pigmented strains, this is below 60 lux, and for albino strains it is below 5 lux^3,4. While the guidelines help to minimize retinal phototoxicity, light levels are influenced greatly by the factors noted above, and the illumination level that is most important is the actual in-cage illumination. Retinal degeneration can take place, especially in albino rodents, even at very low lux levels⁵. Many research investigators who work on retinal light damage and neuroprotection recommend dim to very dim light levels during the light cycle. To avoid confounding effects, they use light levels of 5 lux to 20-30 lux⁶.

Does light intensity vary with rack style?

It is important to keep lighting intensity at appropriate levels and to minimize the intensity variability among the cages on a rack in order to minimize experimental variability. Two studies were undertaken to compare light levels in two types of mice and rat racks; intra-rack variability in light exposure was compared between a single-sided library-style mouse and rat rack with Optimice and Optirat Plus carousel racks, respectively. Compared to the library-style rack, both carousel racks produced less variability in light exposure between cages, and the light intensity levels of the carousel racks were lower overall than in library-style cages. Moreover, light exposure within individual carousel rack cages approached or surpassed standards suggested for pigmented and non-pigmented rodents. Providing a stable and less intense lighting environment for animals is highly recommended⁷, and both the Optimice and Optirat Plus carousel racks effectively meet both conditions. The carousel racks also alleviate the concern that cages at the bottom of the rack or cages in the back against a wall are not exposed to ample light when, in fact, keeping light levels low appears to be preferred by laboratory mice.

Can cages near the top of some rack styles increase stress levels?

A highly relevant epidemiological survey on male mice aggression found several predictors of fighting, among which were cage location on the rack and rack orientation in the room. Mice housed on higher rows in the rack showed evidence of increased stress, including changes in stress physiology, immune function, and abnormal behavior⁸. The authors suggest that animals on higher rows may be exposed to greater noise and vibration in traditional IVC racks (due to the proximity to the fans and blowers), and to brighter lights. For instance, light levels at the top of a rack can be 80 times brighter than at the bottom of the rack¹.

Does the cage or rack location relative to the room wall increase stress levels?

The above study also found that library-style racks oriented parallel to the wall may be more exposed to bright light. But notably, cages on these racks are also exposed to more human traffic; however, in racks oriented perpendicular to the wall, only the outermost cages are exposed to similar levels of human traffic. And exposure to human traffic is related to abnormal behavior in other species⁸. So, if a cage on a carousel rack is positioned in the corner or against a wall, this reduces not only exposure to bright light, but also exposure to human traffic, which is a clear welfare advantage, especially for delicate transgenic mice or sensitive new litters.

How can I help provide a less stressful environment for my mice?

First measure your lights — a lux meter can be purchased inexpensively. To understand the light levels that the mice will experience, place the sensor in an empty cage, especially at the front and at the back of the cage and measure light levels at every row on the rack.

If your lux levels are too high, there are several ways to help lessen the intensity for your mice:

Control light at the source: Dim the lights in the housing room; remove every other tube (or similar) from the ceiling light ballasts or purchase opaque perforated sleeves that help reduce the light.

Control light at the rack level: Avoid housing cages on the rack with the highest light exposure, usually the top rows.