Lighting Research Center

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Light & Health

AlertnessA New Pathway to Alertness

Scientists have long agreed that retinal light exposure regulates our body's circadian rhythms, such as the sleep/wake cycle. LRC scientists recently uncovered new information that may chart a new, additional course in our quest to understand how light impacts human health and well-being.

It is well accepted that the circadian system is maximally sensitive to short-wavelength (blue) light and is quite insensitive to long-wavelength (red) light. LRC scientists found that not only blue light but also moderate levels of red light increased alertness at night. This study suggests that the circadian system is not the only light-sensitive pathway involved in determining alertness at night, and that other mechanisms may exist.


Fourteen subjects participated in a within-subject two-night study. Both nights, the subjects were each presented a high (40 lx) and a low (10 lx) diffuse light exposure condition of the same spectrum. Prior to each lighting condition, the subjects remained in the dark (<1lx) for 60 minutes. Electroencephalograph (EEG) measurements, electrocardiogram (ECG), psychomotor vigilance tests (PVT), self-reports of sleepiness, and saliva samples for melatonin assays were collected at the end of each dark and light period.


  • Exposure to red and to blue light resulted in EEG rhythmic activity changes reflecting increased alertness (increased beta power frequency) and a reduction in relaxation (reduced alpha power frequency) relative to preceding dark conditions.
  • Exposures to high, but not to low, levels of red and blue light significantly increased heart rate relative to the dark conditions.
  • Only the higher level of blue light resulted in a reduction in melatonin levels relative to the other lighting conditions.
  • Performance and sleepiness ratings were not strongly affected by the lighting conditions.



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