Light exposure on alertness after wake-up in healthy men : comparing dim, bright, red, and blue light
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Abstract
Introduction: Light is a key factor in moderating human
alertness, both subjective and objective. However, the
methodology applies in research on the effects of exposure to
light of different wavelengths and intensities on objective and
subjective alertness varies greatly and evidence on objective
alertness in particular is still inconclusive. Thus, the present,
highly standardized within-subject laboratory study on N = 44
healthy males explored how LED light of different intensities
(dim vs. bright light) and wavelengths (red vs. blue) affected
objective (reaction time/RT) as well as subjective (sleepiness)
alertness in the morning after wake-up. Methods: Participants
spent two separate nights in the laboratory and were exposed
to either one of the two light intensities or colors for 60 min
after wake-up. Additionally, they indicated their sleepiness on
the Karolinska Sleepiness Scale and participated in an audi-
tory RT task before and after light intervention. It was hy-
pothesized that both bright and blue light would lead to
greater subjective and objective alertness when compared to
dim and red light, respectively. Results: Results indicated that
average RTs were longer for participants in the bright light
condition (p = 0.004, f2 = 0.07) and that RTs decreased post-
light exposure irrespective of light being dim or bright (p =
0.026, f2 = 0.07). However, dim versus bright light and RT did
not interact (p = 0.758, f2 = 0.07). Chronotype was a significant
covariate in the interaction of dim versus bright light and
subjective sleepiness (p = 0.008, f2 = 0.22). There was no
difference in RTs when comparing exposure to red or blue
light (p = 0.488, f2 = 0.01). Findings on subjective sleepiness
and light of different wavelengths revealed that sleepiness
was reduced after light exposure (p = 0.007, f2 = 0.06), al-
though the wavelength of light did not appear to play a role
in this effect (p = 0.817, f2 = 0.06). Conclusion: Hence, neither
of the hypotheses could be confirmed. However, they indi-
cated that evening types might benefit from exposure to
bright light regarding sleepiness, but not morning types