Blue Light & Caffeine Article

8122019 Blue Light amp Caffeine Article

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A Comparison of Blue Light and Caffeine Effects onCognitive Function and Alertness in Humans

C Martyn Beaven 1 Johan Ekstroumlm 2

1 Swedish Winter Sports Research Centre Department of Health Sciences Mid Sweden University Oumlstersund Sweden 2 Department of Psychology Mid

Sweden University Oumlstersund Sweden

Abstract

The alerting effects of both caffeine and short wavelength (blue) light have been consistently reported The ability of blue light to enhance alertness and cognitive function via non-image forming neuropathways have been suggestedas a non-pharmacological countermeasure for drowsiness ac ross a range o f occupational settings Here we c ompareand contrast the alerting and psychomotor effects of 240 mg of caffeine and a 1-h dose of ~40 lx blue light in a non-athletic population Twenty-one healthy subjects performed a computer-based psychomotor vigilance test before andafter each of four randomly assigned trial conditions performed on different days white lightplacebo white light240mg caffeine blue lightplacebo blue light240 mg caffeine The Karolinska Sleepiness Scale was used to assesssubjective measures of alertness Both the caffeine only and blue light only conditions enhanc ed accuracy in a visualreaction test requiring a decision and an additive effect was observed with respect to the fastest reaction timesHowever in a test of executive function where a distraction was included caffeine exerted a negative effect onaccuracy Furthermore the blue light only condition consistently outperformed caffeine when both congruent andincongruent distractions were presented The visual reactions in the absence of a dec ision or distraction were alsoenhanced in the blue light only condition and this effect was most prominent in the blue-eyed participants Overallblue light and caffeine demonstrated distinct effects on aspects of psychomotor function and have the potential topositively influence a range of settings where cognitive function and alertness are important Specifically despite thewidespread use of caffeine in competitive sporting environments the possible impact of blue light has received noresearch attention

Citation Beaven CM Ekstroumlm J (2013) A Comparison of Blue Light and Caffeine Effects on Cognitive Function and Alertness in Humans PLoS ONE8(10) e76707 doi101371journalpone0076707

Editor Denis Burdakov MRC - NIMR United Kingdom

Received June 7 2013 Accepted August 24 2013 Published October 7 2013

Copyright copy 2013 Beaven Ekstroumlm This is an open-access article distributed under the terms of the Creative Commons Attribution License whichpermits unrestricted use distribution and reproduction in any medium provided the original author and source are credited

Funding The authors have no support or funding to reportCompeting interests The authors have declared that no competing interests existE-mail MartynBeavenmiunse

Introduction

Much like the ear has functions for hearing and balance thehuman eye has a dual role in detecting light for a range of behavioural and physiological responses that are distinct fromtheir role in vision [1] These non-image forming light effectsare known to modulate multiple endocrine behavioural andphysiological responses including melatonin suppression heart

rate elevation enhancement of alertness mood andperformance and stimulation of gene expression [2-9] Thenon-visual alerting effects of light have been s hown to bemediated at least in part by a melanopsin-dependentphotoreceptive system which is specifically sensitive to bluelight (wavelengths of 460-480 nm) [1011]

The alerting response to blue light is effective in reducingsleepiness and enhancing cognitive performance andspecifically in tasks associated with concentration andcognition [12-14] Indeed the ability of short light wavelengths

to modulate alertness via the suppression of melatonin is wellestablished [1516] with greater suppression noted in light eyedCaucasians [17] However the acute suppression of melatoninis not requisite for light to have an effect on measures of alertness at night [18] and short wavelength light has beendemonstrated to elicit enhanced alertness and cognitiveperformance during daylight hours even when the contributionof melatonin suppression is indubitably negligible [19]

Caffeine is a methylxanthine well known to improve mood[2021] vigilance and alertness [2223] via acceleration of motor processes through central andor peripheral mechanisms[2425] The beneficial psychomotor performance effects of caffeine can be regarded in some respects as having similar effects to light of short wavelengths However day-time blue-enriched light has been reported to have a positive effect onsubsequent sleep which contrasts with the negative effects onsleep that have been reported with caffeine administration [26]This would indicate that the effects of blue light and caffeine

PLOS ONE | wwwplosoneorg 1 October 2013 | Volume 8 | Issue 10 | e76707



although comparable are distinct under differentcircumstances

The intention of the current study was to compare andcontrast the physiological responses to blue light and caffeineadministered both separately and conjointly Measures of cognitive function reaction time and wakefulness wereassessed and it was hypothesized that similarities would beobserved with the administration of 240 mg of caffeine and a 1h dose of ~40 lx blue light Further it was hypothesized that acombination of blue light exposure and caffeine ingestion wouldinduce alerting and psychomotor effects greater than either intervention in isolation and that eye colour would influence thedegree of the psychomotor and physiological responses to bluelight

Methods

Ethics StatementThe investigation was conducted according to the principles

expressed in the Declaration of Helsinki All participants

provided informed written consent prior to participation withpre-approval obtained from the Umearing Regional Ethical ReviewBoard ( 2012-318-31M)

ParticipantsThe study recruited 24 subjects (13 male 11 female 26 plusmn 4

y) Individuals were excluded if they had worked night shiftsduring the last 12 months or had travelled through more thanone time zone during the last 2 months All participants werenon-smokers low to moderate caffeine and alcohol consumersand were not on contraindicatory medication Colour blindsubjects were not excluded as normal trichromatic vision is notnecessary for light-mediated neuroendocrine regulation [27]

Experimental ProtocolParticipants reported to the Oumlstersund test facility (631deg N)

on four separate test sessions within a one-month periodduring winter (February-March) All participants refrained fromdrinking caffeinated products or alcohol and strenuousphysical exercise on trial days Prior to testing the participantswere familiarized with a computer-based psychomotor vigilancetest protocol (PVT) and eye colour was assessed visually Onfour intervention days participants arrived at the test facility inthe evening (1700 to 1800 h) after at least a two hour fast andcompleted the PVT (described below) that consisted of 20 trialsof a visual and audio GoNo-Go test an Eriksen Flanker testand 5 trials of a visual reaction time task (all tasks are availableonline at wwwcognitivefunnet) The PVT lasted approximately10 minutes

After performing the PVT the participants ingested a gelatinecapsule containing either 240 mg of caffeine or a visuallyindistinguishable sugar placebo with a small glass of waterThey then rated their level of alertness using the KarolinskaSleepiness Scale (KSS) before entering a separate roomspecifically set up to deliver either ~40 lx of blue light from aLED light source (Techlightreg RGB 3W λmax = 470 nm) or awhite light alternative (~100 lx) for 1 h Thus there were four

trial conditions administered in a random manner to avoidsequence effects white lightplacebo (PLA) white light240 mgcaffeine (CAF) blue lightplacebo (BLU) blue light240 mgcaffeine (BCAF) Each trial was of equivalent duration andlasted 1 h during which the participants were consistentlyexposed to the light stimulus and were instructed to keep their eyes open while remaining comfortably seated and listening torelaxing music Every 15 minutes a researcher entered theroom to re-evaluate the level of alertness using the KSS andensure subject compliance At the end of the 1 h lightexposure the participants again completed the PVT

Psychomotor Vigilance Test (PVT) The first test of thePVT was a GoNo-Go task which has previously been used toassess sustained attention [1214] The GoNo-Go Testinvolved the random presentation of 20 visual stimuli and then20 audio stimuli and the participant was requ ired to strike thespace bar on the computer keyboard (Go) or inhibit that action(ignore the stimulus) when a non-cued stimuli was presented(No-Go) The fastest and average reaction times wererecorded for analysis

The Eriksen Flanker task is an executive function test usedin cognitive psychology to assess psychomotor vigilance andthe ability to suppress responses that are inappropriate in aparticular context In the task a directional response was madeto a central target stimulus by striking the left or right arrow onthe computer keyboard The target was randomly flanked bynon-target stimuli which corresponded either to the samedirectional response as the target ( congruent flankers) or to theopposite response ( incongruent flankers) The fastest andaverage reaction times for both the congruent and incongruentpresentations were recorded for analysis

The visual reaction time task consisted of five presentationsof a visual stimulus on the computer screen where theparticipant was instructed to strike the space bar as quickly as

possible upon presentation The fastest reaction time wasrecorded for analysis The GoNo-Go Eriksen Flanker andvisual reaction time tasks were all performed on the samecomputer installed with the flux program (available online atwwwstereopsiscom) to minimise extraneous blue lightemission that has been demonstrated to influence psychomotor performance [14]

Karolinska Sleepiness Scale (KSS) The KSS was used tosubjectively assess alertness as this test is used for evaluatingsubjective sleepiness [28] and has been reported to be aldquouseful proxyrdquo for electroencephalographic (EEG) activity [29]The scale has anchor points which range from 1 (very alert) to9 (very sleepy an effort to stay awake fighting sleep)

Statistical Analyses An analysis of variance with factors for caffeine dose and

light treatment was utilized to determine differences in themeans of the response variables resulting from the four treatments Differences were interpreted in relation to thelikelihood of exceeding the smallest worthwhile effect withindividual change thresholds for each variable Variables werelog- or arsineroot-transformed when appropriate to reduce biasarising from non-uniformity of error Magnitudes of thestandardized effects (Cohen Effect Size [ES]) were interpreted

Blue Light and Caffeine Effects in Humans




using thresholds of 02 06 12 and 20 for small moderatelarge and very large respectively [30] Standardized effects of between -019 and 019 were termed trivial Quantitativechances of higher or lower differences were evaluatedqualitatively as follows lt1 almost certainly not 1ndash5 veryunlikely 5ndash25 unlikely 25ndash75 possible 75ndash95 likely95ndash99 very likely gt99 almost certain To make inferencesabout the large-sample value of an effect the uncertainty in theeffect was expressed as 90 confidence limits (CL) An effectwas deemed unclear if the confidence interval overlapped thethresholds for both small positive and small negative effectsThe significance level was set at ple 005

Results

Twenty-one participants (13 male 8 female) completed theentire experimental procedure with an average relative caffeinedose of 32 plusmn 05 mgkg The number of correct responses inthe visual GoNo-Go task decreased in the PLA condition(Figure 1) with a clear difference between the positive effect of CAF (ES 066) and BLU (ES 050) No accuracy differences( correct) were observed between the conditions in the audioGoNo-Go task Post-treatment differences in the GoNo-Gotask were observed between the average audio reaction times(BLU lt CAF ES 049) and for the fastest visual reaction time(BCAF lt PLA ES 043)

The BCAF condition caused an improvement in the fastestvisual GoNo-Go reaction time task (Figure 2) that was superior to PLA (-87 plusmn 71 ES 072) CAF (-44 plusmn 60 ES 044)and BLU (-53 plusmn 53 ES 045)

With respect to the Eriksen Flanker task there was asignificant difference between the congruent and incongruentreaction times (359 vs 382 ms p = 00141) Caffeine ingestionhad a large negative impact on the percentages of correctresponses in the incongruent presentation (Figure 3) In thecongruent presentations the only observed difference in the

Figure 1 Change in the accuracy in the visual GoNo- Gopsychomotor task a small difference from PLA b moderatedifference from PLA PLA white lightplacebo CAF whitelight240 mg caffeine BLU blue lightplacebo BCAF bluelight240 mg caffeinedoi 101371journalpone0076707g001

change from pre- to post-intervention was a small accuracyimprovement in the PLA condition compared to the BCAF

condition (ES 053) The change in performance (reactiontime) in the Eriksen Flanker task is depicted in Figure 4 Theblue light treatment consistently showed beneficialperformance effects when compared to the CAF condition

Males had faster reaction times than females in the visualreaction time task (255 vs 274 ms p = 00172) Post-treatmentdifferences in reaction times were observed between theconditions PLA gt BLU (63 plusmn 52 ES 065) PLA gt CAF (55plusmn 56 ES 053) and BCAF gt BLU (42 plusmn 46 ES 047)

Figure 2 Change in performance of the visual GoNo- Gopsychomotor task a small difference from BLU b smalldifference from CAF c moderate difference from PLA PLAwhite lightplacebo CAF white light240 mg caffeine BLUblue lightplacebo BCAF blue light240 mg caffeinedoi 101371journalpone0076707g002

Figure 3 Change in the accuracy in in congru entpresentations in the Eriksen flanker psychomotor task alarge difference from CAF b large difference from BCAF cmoderate difference from CAF d moderate difference fromBCAF PLA white lightplacebo CAF white light240 mgcaffeine BLU blue lightplacebo BCAF blue light240 mgcaffeinedoi 101371journalpone0076707g003





The changes in reaction times from pre- to post-interventionare presented in Figure 5

Interestingly the improved reaction times seen after the bluelight treatment in Figure 5 (ES 041 p = 00410) were drivenby the blue-eyed participants (n = 14) with no difference inreaction time observed in the non-blue-eyed participants (n = 7p = 03840) Similarly within the BCAF treatment there was aclear difference between the response of the blue-eyed andnon-blue-eyed participants (58 plusmn 65 ES 050)

The mean pre-treatment subjective alertness levels were 41plusmn 15 units The changes in subjective measures of alertnessresulting from the experimental conditions are presented in

Figure 4 Change in perform ance of the Eriksen flanker psychomotor task a small difference from PLA b smalldifference from CAF c moder ate differ ence from CAF PLAwhite lightplacebo CAF white light240 mg caffeine BLUblue lightplacebo BCAF blue light240 mg caffeine CON

Congruent flanker presentations INCON Incongruent flanker presentationsdoi 101371journalpone0076707g004

Figure 5 Change in perform ance of the visual reactiontime task a small difference from PLA PLA white lightplacebo CAF white light240 mg caffeine BLU blue lightplacebo BCAF blue light240 mg caffeinedoi 101371journalpone0076707g005

Figure 6 No relationships were observed between the post-treatment KSS or the change in KSS scores and any measuresof cognitive function or psychomotor ability

Discussion

Here we demonstrate for the first time that blue light andcaffeine demonstrate distinct effects on aspects of psychomotor function presumably via distinct mechanismsPreviously a combination of bright white light and caffeine has

been assessed as a countermeasure for impaired alertnessand performance during extended sleep deprivation [31]Interestingly the effects of the blue light treatment in our visualreaction time task were mediated in part by the eye colour of the participants Thus we contend that the previously observedinfluence of eye pigmentation on wavelength-specific melatoninsuppression [17] can be extended to psychomotor effects andfurther that this effect is independent of ethnicity

Both blue light exposure and caffeine ingestion improvedaccuracy in the visual GoNo-Go task however their combination did not result in enhancement in the number of correct responses (Figure 1) It has been proposed that theeffects of caffeine follow the Yerkes-Dodson law in which it ispostulated that the relationship between arousal andperformance follows an inverted U-shape curve [24] Thus if acting upon similar brain architecture it is possible torationalize that the combined treatment of blue light andcaffeine dose exceeded the optimal state of arousal andconsequently resulted in impaired accuracy However thecombination of blue light with caffeine did result in animprovement in the fastest reaction time in the visual GoNo-Go task in line with previous research using short wavelengthlight [1214] which suggests an additive effect of the treatments(Figure 2)

Figure 6 Change in Karolinska sleepiness scale ratingsover time a small difference from PLA b small differencefrom BLU c small difference from BCAF d moderatedifference from BCAF PLA white lightplacebo CAF whitelight240 mg caffeine BLU blue lightplacebo BCAF bluelight240 mg caffeine AU Arbitrary unitsdoi 101371journalpone0076707g006





As seen in previous research the response times in theEriksen Flanker task were slower for incongruent stimuli thanfor congruent stimuli [32] Also similar to earlier research [12]we found no little or no difference between the blue lighttreatment and our placebo condition in the Eriksen Flanker taskdesigned to assess executive function (Figures 3 and 4)

Accuracy data did howe ver demonstrate a clear decrease inaccurate responses to incongruent presentations when caffeinewas administered (Figure 3) It is possible that this decrease inperformance is related to the anxiogenic properties of caffeineWhile caffeine is known to improve alertness and can improvecognitive and motor function performance [2431] studies havereported negative effects of caffeine consumption on handsteadiness [33] and anxiety [34] While there was a smalldifference between the PLA and BLU treatments in the fastestreaction time in the congruent presentation the blue lighttreatment consistently outperformed CAF in all tasks (Figure 4)

Our subjective sleepiness data derived from the KarolinskaSleepiness Scale demonstrated that the well known effect of caffeine on the participant srsquo per ception of alertness occurred

quite rapidly (Figure 6) However we did not see a similar effect on subj ective alertness in either the BLU or BCAFtreatment groups Previous researchers have reporteddecreases in both objective and subjective measures of sleepiness as a result of short wavelength light exposure[114] although non-significant effects of light treatment havealso been observed [18] It is worth noting then that thealerting effects of the blue light treatment highlighted aboveoccurred independently of subjective sleepiness which mayhave important ramifications given the known negative impactof caffeine on sleep latency and quality [35]

Although caffeine is known to improve alertness and canimprove cognitive and motor function performance [2431] aswell as athletic skill execution [36] numerous studie s r eport

negative effects of caffeine consumption with impacts on handsteadiness [33] sleep latency and quality [35] and anxiety [34]Caffeine administration (2 mgkg) has also been associatedwith impaired recall in male subjects presented with a 12-itemword list [37] It has been suggested that many of the effects of caffeine constitute a return to normal function or anamelioration of the effects of caffeine withdrawal and thusdoes not improve psychomotor function above lsquonormalrsquo levels[38] However a number of researchers have demonstratedthat caffeine has psychoactiv e effects even in the absence of withdrawal [39-41] It should also be noted that some of theeffects of caffeine have been attributed to expectancy [42] andthat despite the double-blinded study design we cannotdiscount the possibility that some participants were consciousof the physiological stimulatory actions of caffeine and adjustedtheir subsequent behaviours accordingly Further individualresponses to caffeine are co mmonly reported in literature and ithas been suggested that individual differences in sensitivityresult from intrinsic differences in responsiveness to caffei ne atsites of action in the brain rather than from differences inabsorption distribution or metabolism of the substance [43]

The light intensity in the current study of ~40 lx has beenused previously to induce an alerting effect after a 1 hexposure [44] The difference in the light intensity between the

PLA and BLU treatments demonstrates that any alerting effectsobserved in the blue light treatment group were as a result of the wavelength of light and independent of light intensity Thecurrent study utilised an exposure time of 1 h in order to matchthe duration of effect with caffeine which is absorbed anddemonstrates physiological effects after 45-60 min whenadministered orally [24] It is known however that just 50 s of short-wave light exposure can cause detectable effects in thehypocampus and amygdala brain areas associated witharousal [45] and thus can affect cognitive functions almostinstantaneously Further the exposure time and intensity hasbeen suggested to influence the nature of the response to lighttreatment the persistence of the response and the brainregions affected [114546]

A dual-pathway has been hypothesized to explain thephysiological and psychological outcomes of blue lightexposure [47] A limbic pathway that involves the amygdala ishypothesized to work in parallel with the circadian pathway(retinohypothalamic tract) to elicit the light-inducedimprovements in subjective alertness independent of melatoninsuppression M RI studies have shown that daytime exposure toblue light when compared with green almost instantaneouslyenhances brain responses to a memory task in several corticalthalamic and brainstem areas [48] The direct projection to theamygdala from melanop sin-expressing photosensitive ganglioncells is believed to explain the rapid limbic responses to lightwith subsequent modulation of brain activity in cortical areasupon prolonged exposure [ 46]

The alerting effects of short-wavelength light have led to thesuggestion that light can be used as a non-p harmacologicalcountermeasure in a range of occupational settings [1]However the potential for light therapy to impact athleticperformance has been overlooked Here we showimprovements in reaction time as a result of blue light exposureand such improvements may have athletic implicationsespecially given recent research demonstrating a link betweenreaction time and sprint performan ce [49] Further sincereaction time is an important component of agility [50] bluelight has the potential to enhance athletic performance asnumerous sports are performed indoors with artificial lightingconditions and or at night time

Here we combined for the first time the known alertingeffects of blue light exposure with the widely used stimulantcaffeine and demonstrated some divergent effects of the twotreatments In instances where a rapid decision in response toa visual stimulus was required additive positive effects were

observed however caffeine demonstrated negative effects ontasks of executive function with an inability to suppressinappropriate responses in the context of our Er iksen Flanker task Indeed blue light consistently improved executivepsychomotor function when compared to oral ca ffeineadministration Blue light also demonstrated positive effects onvisual reaction time an effect that was more pronounced inblue-eyed participants This alerti ng effect could have benefitsin a variety of occupational and contexts including competitivesporting environs





Author Contributions

Conceived and designed the experiments CMB JE Performedthe experiments CMB JE Analyzed the data CMB JE

Contributed reagentsmaterialsanalysis tools CMB JE Wrotethe manuscript CMB JE

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although comparable are distinct under differentcircumstances

The intention of the current study was to compare andcontrast the physiological responses to blue light and caffeineadministered both separately and conjointly Measures of cognitive function reaction time and wakefulness wereassessed and it was hypothesized that similarities would beobserved with the administration of 240 mg of caffeine and a 1h dose of ~40 lx blue light Further it was hypothesized that acombination of blue light exposure and caffeine ingestion wouldinduce alerting and psychomotor effects greater than either intervention in isolation and that eye colour would influence thedegree of the psychomotor and physiological responses to bluelight

Methods

Ethics StatementThe investigation was conducted according to the principles

expressed in the Declaration of Helsinki All participants

provided informed written consent prior to participation withpre-approval obtained from the Umearing Regional Ethical ReviewBoard ( 2012-318-31M)

ParticipantsThe study recruited 24 subjects (13 male 11 female 26 plusmn 4

y) Individuals were excluded if they had worked night shiftsduring the last 12 months or had travelled through more thanone time zone during the last 2 months All participants werenon-smokers low to moderate caffeine and alcohol consumersand were not on contraindicatory medication Colour blindsubjects were not excluded as normal trichromatic vision is notnecessary for light-mediated neuroendocrine regulation [27]

Experimental ProtocolParticipants reported to the Oumlstersund test facility (631deg N)

on four separate test sessions within a one-month periodduring winter (February-March) All participants refrained fromdrinking caffeinated products or alcohol and strenuousphysical exercise on trial days Prior to testing the participantswere familiarized with a computer-based psychomotor vigilancetest protocol (PVT) and eye colour was assessed visually Onfour intervention days participants arrived at the test facility inthe evening (1700 to 1800 h) after at least a two hour fast andcompleted the PVT (described below) that consisted of 20 trialsof a visual and audio GoNo-Go test an Eriksen Flanker testand 5 trials of a visual reaction time task (all tasks are availableonline at wwwcognitivefunnet) The PVT lasted approximately10 minutes

After performing the PVT the participants ingested a gelatinecapsule containing either 240 mg of caffeine or a visuallyindistinguishable sugar placebo with a small glass of waterThey then rated their level of alertness using the KarolinskaSleepiness Scale (KSS) before entering a separate roomspecifically set up to deliver either ~40 lx of blue light from aLED light source (Techlightreg RGB 3W λmax = 470 nm) or awhite light alternative (~100 lx) for 1 h Thus there were four

trial conditions administered in a random manner to avoidsequence effects white lightplacebo (PLA) white light240 mgcaffeine (CAF) blue lightplacebo (BLU) blue light240 mgcaffeine (BCAF) Each trial was of equivalent duration andlasted 1 h during which the participants were consistentlyexposed to the light stimulus and were instructed to keep their eyes open while remaining comfortably seated and listening torelaxing music Every 15 minutes a researcher entered theroom to re-evaluate the level of alertness using the KSS andensure subject compliance At the end of the 1 h lightexposure the participants again completed the PVT

Psychomotor Vigilance Test (PVT) The first test of thePVT was a GoNo-Go task which has previously been used toassess sustained attention [1214] The GoNo-Go Testinvolved the random presentation of 20 visual stimuli and then20 audio stimuli and the participant was requ ired to strike thespace bar on the computer keyboard (Go) or inhibit that action(ignore the stimulus) when a non-cued stimuli was presented(No-Go) The fastest and average reaction times wererecorded for analysis

The Eriksen Flanker task is an executive function test usedin cognitive psychology to assess psychomotor vigilance andthe ability to suppress responses that are inappropriate in aparticular context In the task a directional response was madeto a central target stimulus by striking the left or right arrow onthe computer keyboard The target was randomly flanked bynon-target stimuli which corresponded either to the samedirectional response as the target ( congruent flankers) or to theopposite response ( incongruent flankers) The fastest andaverage reaction times for both the congruent and incongruentpresentations were recorded for analysis

The visual reaction time task consisted of five presentationsof a visual stimulus on the computer screen where theparticipant was instructed to strike the space bar as quickly as

possible upon presentation The fastest reaction time wasrecorded for analysis The GoNo-Go Eriksen Flanker andvisual reaction time tasks were all performed on the samecomputer installed with the flux program (available online atwwwstereopsiscom) to minimise extraneous blue lightemission that has been demonstrated to influence psychomotor performance [14]

Karolinska Sleepiness Scale (KSS) The KSS was used tosubjectively assess alertness as this test is used for evaluatingsubjective sleepiness [28] and has been reported to be aldquouseful proxyrdquo for electroencephalographic (EEG) activity [29]The scale has anchor points which range from 1 (very alert) to9 (very sleepy an effort to stay awake fighting sleep)

Statistical Analyses An analysis of variance with factors for caffeine dose and

light treatment was utilized to determine differences in themeans of the response variables resulting from the four treatments Differences were interpreted in relation to thelikelihood of exceeding the smallest worthwhile effect withindividual change thresholds for each variable Variables werelog- or arsineroot-transformed when appropriate to reduce biasarising from non-uniformity of error Magnitudes of thestandardized effects (Cohen Effect Size [ES]) were interpreted






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Documents

Blue Light & Caffeine Article