Red light at intensities above 10 lx alters sleep–wake behavior in mice

Sleep is regulated by two mechanisms: the homeostatic process and the circadian clock. Light affects sleep and alertness by entraining the circadian clock, and acutely inducing sleep/alertness, in a manner mediated by intrinsically photosensitive retinal ganglion cells. Because intrinsically photosensitive retinal ganglion cells are believed to be minimally sensitive to red light, which is widely used for illumination to reduce the photic disturbance to nocturnal animals during the dark phase. However, the appropriate intensity of the red light is unknown. In the present study, we recorded electroencephalograms and electromyograms of freely moving mice to investigate the effects of red light emitted by light-emitting diodes at different intensities and for different durations on the sleep–wake behavior of mice. White light was used as a control. Unexpectedly, red light exerted potent sleep-inducing effects and changed the sleep architecture in terms of the duration and number of sleep episodes, the stage transition, and the EEG power density when the intensity was >20 lx. Subsequently, we lowered the light intensity and demonstrated that red light at or below 10 lx did not affect sleep–wake behavior. White light markedly induced sleep and disrupted sleep architecture even at an intensity as low as 10 lx. Our findings highlight the importance of limiting the intensity of red light (⩽10 lx) to avoid optical influence in nocturnal behavioral experiments, particularly in the field of sleep and circadian research. Sleep behaviour: Red light intensity matters for sleep High-intensity red light is comparable to white light in inducing sleep behaviours in mice. Red light is used to mimic darkness in lab-based nocturnal experiments because retinal cells have a low sensitivity to it. However, variations in data from similar experiments may be a result of using different intensities of red light. By recording the brain and muscle activity of mice, Huang and colleagues from Fudan University, China, examined the effect of different red light intensities on sleep behaviour and compared it with that induced by white light. They found that, like white light, red light intensities over 20 lux induced sleep, whereas intensities below 10 lux did not affect sleep−wake behaviour. These findings provide important guidelines for nocturnal behavioural experiments on rodents and highlight the importance of light intensity for effective sleep.