Which of the following is a recommended practice for lighting in rooms housing rodents?

• A. Expose animals to bright light throughout the night
• B. Use a consistent 12:12 hr light-dark cycle
• C. Turn lights on and off randomly
• D. Place lights directly overhead to maximize visibility

Answer: (B)

Maintaining a stable, predictable light-dark cycle is essential because rodents’ physiology and behavior follow circadian rhythms tied to environmental light. A consistent 12-hour light/12-hour dark schedule gives them reliable cues for activity, feeding, and hormonal cycles, supporting normal welfare and more reliable experimental outcomes. Exposing animals to bright light throughout the night disrupts these rhythms, elevates stress, and can alter metabolism and immune function, which may skew data. Turning lights on and off randomly also interrupts the daily cycle, causing stress and unpredictable responses. While lighting direction matters for visibility, the key point is not to disturb the cycle; the best practice is to provide a regular 12:12 cycle with minimal light leakage during the dark phase.