Alerting effects of short-wavelength (blue) and long-wavelength (red) lights in the afternoon

Levent Sahin, Mariana G. Figueiro

Research output: Contribution to journalArticlepeer-review

133 Scopus citations


Light has an acute effect on neuroendocrine responses, performance, and alertness. Most studies to date have linked the alerting effects of light to its ability to suppress melatonin, which is maximally sensitive to short-wavelength light. Recent studies, however, have shown alerting effects of white or narrowband short-wavelength lights during daytime, when melatonin levels are low. While the use of light at night to promote alertness is well understood, it is important to develop an understanding of how light impacts alertness during the daytime, especially during the post-lunch hours. The aim of the current study was to investigate how 48-minute exposures to short-wavelength (blue) light (40lux, 18.9microWatts/cm2 λmax=470nanometers [nm]) or long-wavelength (red) light (40lux, 18.9microWatts/cm2 λmax=630nm) close to the post-lunch dip hours affect electroencephalogram measures in participants with regular sleep schedules. Power in the alpha, alpha theta, and theta ranges was significantly lower (p<0.05) after participants were exposed to red light than after they remained in darkness. Exposure to blue light reduced alpha and alpha theta power compared to darkness, but these differences did not reach statistical significance (p>0.05). The present results extend those performed during the nighttime, and demonstrate that light can be used to increase alertness in the afternoon, close to the post-lunch dip hours. These results also suggest that acute melatonin suppression is not needed to elicit an alerting effect in humans.

Original languageEnglish
Pages (from-to)1-7
Number of pages7
JournalPhysiology and Behavior
StatePublished - 7 May 2013
Externally publishedYes


  • Alertness
  • Electroencephalogram (EEG)
  • Intrinsically photosensitive retinal ganglion cells
  • Light
  • Melatonin
  • Post-lunch dip
  • Sleepiness


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