Here we demonstrate that the alerting response to polychromatic light in the evening is wavelength-dependent, such that light at 6500K is more effective than light at 2500K and 3000K in reducing subjective sleepiness and enhancing cognitive performance, specifically associated with tasks of sustained attention.
In our study, light exposure caused a wavelength-dependent suppression of salivary melatonin, such that light at 6500K resulted in a significant attenuated melatonin secretion, particularly after 90 minutes of light exposure, and which persisted during post-light exposure. This stands in agreement with recent findings suggesting that the human circadian pacemaker is highly sensitive to short wavelength light 
, as indexed by action spectra for human melatonin suppression and assessment of human circadian phase resetting 
. The differential spectral sensitivity of non-image forming responses to visual responses 
has challenged the classical involvement of rod and cone photopigments in responses to light. Since Berson and co-workers 
detected intrinsic photosensitive retinal ganglion cell (ipRGC) in the mammalian retina, it began to emerge that the eye plays a dual role in detecting light for a range of behavioral and physiological responses apart from the classical visual responses. Melanopsin-containing ipRGCs have a specialized non-visual retino-hypothalamic tract which provides direct neuronal connections to the suprachiasmatic nucleus (SCN), as well as direct and indirect (via SCN) projections to brain areas implicated in the regulation of arousal 
. Furthermore, the SCN has connections to the pineal gland, which is responsible for the regulation of melatonin, as well as with many areas that share input from the visual photoreceptor system, such as the lateral geniculate nucleus, pretectum, and superior colliculus 
. However, very recent findings suggest that cone photoreceptors also contribute substantially to non-visual responses at the beginning of a light exposure and at low irradiances, whereas melanopsin may be the primary circadian photopigment in response to long-duration light exposure and at high irradiances 
Our results on nocturnal melatonin decrease after light at 6500K thus go in line with the now established non-visual effects of light on the circadian system. However, while previous studies focused on monochromatic light exposure, we could demonstrate that even with light at 6500K from commercially available compact fluorescent lamps melatonin is suppressed by nearly 40% in comparison to traditional light lamps (3000K). Many studies have shown that exposure to white polychromatic light during night-time increases alertness 
. The dose-response relationship between light alerting effects and its irradiance is such that half of the maximum alerting response to bright light at 9100 lux can be obtained with room light of approximately 100 lux 
. In contrast, our study revealed that light at 6500K from compact fluorescent lamps at low intensity (approximately 40 lux) was most effective after approximately 90 minutes of exposure. Although this luminance level is rather low to elicit such alerting effects, it is likely that the higher amount of blue component in the action spectra of light at 6500K may suffice to elicit these alerting effects. This finding adds up to the high specificity for light in the short wavelength range, and shows that the non-image forming visual system seems to crucially depend on light exposure at particular wavelengths.
Interestingly, we could demonstrate that exposure to blue-enriched light was most effective in enhancing cognitive performance for tasks of sustained attention, while no apparent benefit seemed to occur for higher executive cognitive tasks, such as the PVSAT. Light exposure impacts on cognitive performance through its synchronizing/phase-shifting effects on the circadian clock or acutely via its alerting effects, to the extent that performance in tasks such as digit recall, serial addition-subtraction and simple reaction time tasks can immediately improve after nocturnal light onset 
. Recent fMRI data supports light-induced modulations of cortical activity during auditory cognitive tasks for alertness-related subcortical structures, such as the brainstem 
, the hypothalamus, in a location encompassing the SCN 
, and dorsal and posterior parts of thalamus 
, and amygdala 
. These responses point towards a wide-range of subcortical and cortical networks activated by non-visual effects of light during specific cognitive tasks. While cognitive tasks associated with sustained attention can improve under short-wavelength light exposure, there remains some controversy with respect to higher executive functioning. fMRI assessed brain responses undergo a wavelength-dependency for higher executive task (2-back task), such that blue light (480 nm) enhances modulations in the brainstem (in a LC-compatible location), in the thalamus and insula, in relation to green (550 nm) and violet exposures (430 nm). However, while subcortical regions are activated faster and show short-lasting responses to light, cortical activity requires stronger and longer stimulations, as indicated in a study 
, in which 20 minutes of bright white light induced both thalamic and cortical modulations that steadily declined after light exposure albeit its rather lasting effects.
One critical finding in our study is that lower levels of salivary melatonin were strongly correlated with faster reaction times in tasks of sustained attention (PVT and GO/NOGO), particularly in the light at 6500 K condition. Interestingly, light at 6500K shifted the PVT distribution curve towards the faster range of RT in comparison to light at 2500K and 3000K. Thus, rather the optimal (i.e. 10% fastest RT) than the lapse domain of the PVT, which responds earliest to elevated homeostatic sleep pressure 
was affected by light at 6500K. Similar effects in the optimal domain of the PVT (i.e. quicker 10% fastest RT) were reported after caffeine intake (200 mg), particularly in caffeine sensitive volunteers 
. Thus, from a sustained attention perspective, blue enriched light has similar effects as caffeine such that both treatmens affect the optimal performance domain. In well-rested conditions, this optimal PVT domain has been associated with greater BOLD responses in the fronto-parietal sustained attention network 
. This, in turn, allows for the allocation of attentional resources, by reflecting a top-down modulation of attentional for sustaining focus on a task 
. Light irradiance signal reaches the SCN in the hypothalamus, and then the LC in the brainstem which triggers thalamic activity and a widespread modulation over cortical activity 
. Given the significant inverse correlation of PVT RT with melatonin levels during light exposure at 6500K, one might speculate that enhanced activation in the sustained attentional network may be modulated by the non-visual system via the photopigment melanopsin.
When considering the implications of blue-enriched light on non-clinical settings, it is also important to take into account how this light setting can impact on subjective well-being. In our study, exposure to blue-enriched light improved subjective well-being, which has also been described for blue-enriched daytime light exposure 
. Taken together, our results imply that alertness, subjective well-being and cognitive performance can be improved by enriching the spectral composition of light sources with short wavelengths.
Light-producing devices such as compact fluorescent lamps emitting more short-wavelength energy are effective at stimulating subjective and objective correlates of alertness and performance in the evening, which is correlated with the degree of concomitant suppression of endogenous melatonin levels. Since some of the effects persisted after light exposure, it is fair to suggest that conventional visual photoreception was not the major mediator of these responses. Furthermore, these light settings may provide an effective rationale for enhancing alertness and performance, as well as in the treatment for circadian rhythm sleep disorders, such as shift work disorder.