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Comment
. 2016 Aug 15;14(8):e2000111.
doi: 10.1371/journal.pbio.2000111. eCollection 2016 Aug.

Alerting or Somnogenic Light: Pick Your Color

Patrice Bourgin  1 , Jeffrey Hubbard  2
Affiliations
Comment

Alerting or Somnogenic Light: Pick Your Color

Patrice Bourgin et al. PLoS Biol. .

Abstract

In mammals, light exerts pervasive effects on physiology and behavior in two ways: indirectly through clock synchronization and the phase adjustment of circadian rhythms, and directly through the promotion of alertness and sleep, respectively, in diurnal and nocturnal species. A recent report by Pilorz and colleagues describes an even more complex role for the acute effects of light. In mice, blue light acutely causes behavioral arousal, whereas green wavelengths promote sleep. These opposing effects are mediated by melanopsin-based phototransduction through different neural pathways. These findings reconcile nocturnal and diurnal species through a common alerting response to blue light. One can hypothesize that the opposite responses to natural polychromatic light in night- or day-active animals may reflect higher sensitivity of nocturnal species to green, and diurnals to blue wavelengths, resulting in hypnogenic and alerting effects, respectively. Additional questions remain to be clarified. How do different light wavelengths affect other behaviors such as mood and cognition? How do those results apply to humans? How does light pose either a risk or benefit, depending on whether one needs to be asleep or alert? Indeed, in addition to timing, luminance levels, and light exposure duration, these findings stress the need to understand how best to adapt the color spectrum of light to our needs and to take this into account for the design of daily lighting concepts-a key challenge for today's society, especially with the emergence of LED light technology.

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Conflict of interest statement

The authors have declared that no competing interests exist.

Figures

Fig 1
Fig 1. Summary of light parameters influencing sleep and alertness in humans.
In humans, light exerts strong alerting effects that depend upon several parameters, such as dose (irradiance), duration, and the time of day of light exposure. This photic regulation of alertness also critically depends on the spectral composition of light, with blue light as a powerful alerting stimulus, contrary to green light, which might be less powerful to awaken a subject from sleep.
Fig 2
Fig 2. Light effects in diurnal and nocturnal species: A complex role of the spectral composition.
Light is known to promote sleep in nocturnal species and alertness in day-active animals. Our hypothesis is that the opposite responses to natural polychromatic light may result from higher sensitivity of nocturnal species to green light, and diurnals to blue wavelengths, resulting in hypnogenic and alerting effects, respectively. This can be modeled as different blue and green weights on a balance, explaining either an alerting (higher sensitivity to blue) or sleep-promoting effect (greater sensitivity to green) of white light in diurnal and nocturnal species, respectively.
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