Abstract
The mammalian circadian system regulates all biological processes, thereby ensuring optimal function at the appropriate times of day. Animal studies that examine neurobehavioral processes at different times of day, including during the animal's active phase, may provide important new biomedical insights. A logistical problem for the study of nocturnal laboratory rodents is the potential confounding influence of nighttime light exposure, which may cause circadian disruption and alteration of behavior. The historical solution has been to use red light illumination, which is widely believed to be undetected by the rodent visual system. However, some recent studies have questioned this belief. We, therefore, tested the effects of nighttime exposure to commonly used red light conditions on the circadian non-image forming and the image forming visual systems of female and male laboratory rodents. We found that brief dim red light exposure to a range of red light wavelengths produces strong activation of the suprachiasmatic nucleus master clock, rapid suppression of melatonin secretion, and a subsequent phase shift in daily activity onsets. We also found in an operant behavioral task that rats are able to detect long wavelengths of red light, but not near-infrared light. Thus, both the non-image and image forming visual systems of laboratory rodents are responsive to red light conditions that are often used in animal research. The use of red light for laboratory rodent research and animal care should be carefully considered in terms of its possible confounding influences on research objectives.