- Exposure to bright lightsuppresses melatonin (the sleep promoting hormone)and has circadian phase shifting characteristics [1-3]: It is able to advance or delay our internal body clock, and is naturally used by our body to align theinternal clock with the day/night cycle, thus regulating sleep.
- The human circadian rhythm is most sensitive to short wavelength blue light [1, 2]. When white light is used, 185 times the intensity is requiredcompared to blue light, to achieve the same effect.
- Light exposure and its impact on our physiology is time-dependent [5, 6]: Generally, light exposure in the morning advances the body clock, whereas light in the evening delays the body clock.
- Even 15-20 minutes of exposureto blue light, with intensityas low as 90 corneal lux,is sufficient to cause a shift of thebody clockand reset the sleep/wake cycle.
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- Light therapy is a standard treatment indication by the American Academy of Sleep Medicine (AASM)for treating those suffering from chronobiological aspectsof insomnia. These include:delayed sleep phase syndrome, advanced sleep phase syndrome, irregular sleep/wake cycle, non-24-hour sleep-wake syndrome, jet lag, social jetlag, shift work, dementia, and sleep complaints in the healthy elderly[9, 10].
- A meta study including 53 studies and a total of 1154 participants, found that light therapy was effective in the treatment of sleep problems in general, and forcircadian rhythm sleep disorders, insomnia, and sleep problems related to Alzheimer's disease/dementia specifically .
- A review concludes that there is sufficient evidence to warrant bright light therapy for the treatment of chronic sleep onset and early morning awakening insomnia .
- Especiallyyoung people suffer regularlyfrom insufficient (<7 hours) sleep on weekdaysand have a one to three hour delayin their sleep/wake timing[14, 15].
- This trend tostaying awake in theevening and resulting discrepancy between the body clock and the day/night rhythmis known as social jetlag, andis associated with reduced sleep quality and cognitive performance.
- Bright light therapy in the morning coupled with cognitive behavioural therapy, has been shown toincreasetotal sleep time, result in earlier bed and rise times, and reduce daytime sleepiness[18, 19].
- Increasedlight exposure at night, for example through electronic device usage, suppresses the production of melatonin, keeps the body alertedand thus results in increased sleep onset latency and reducedsleep quality [16, 20].
- However, long-wavelength red light does not suppress melatonin levels and therefore has minimal circadian phase shifting effects .
- Blue depleted ambient light in the evening has been shown to reduce both melatonin suppression and alertness, creating a better sleep environment and allowing the body to prepare for bed.
- It has beenfound that exposure to red lightbefore bed can be highly effective to promote sleepiness and maintain regular sleep/wake cycles.
- Daytime exposure to bright light has been shown to suppress melatonin levels, decrease sleepiness, increase subjective alertness and improve performance on tasks requiring sustained alertness and fast reaction times [24, 25].
- When testing the effect of blue-enriched white light in an office setting, it has been found that increased light exposure in the workplace leads to increased alertness, positive mood, increased performance, reduced irritability and increased concentration. Daytime sleepiness was reduced and participants reported better sleep during the night .
- Light therapy has been shown to raise levels of self-reported wellbeing and vitality in healthy people, particularly during winter time .
- Studies which have tested the efficacy of light therapy on patients suffering from seasonal affective disorder (SAD), found that light therapy caused a faster response and had fewer adverse effectsthan drugs.
- Low-intensity blue-enriched white light was found to be as effective ashigh intensitybright light in treating SAD 
- Lockley, S.W., G.C. Brainard, and C.A. Czeisler, High sensitivity of the human circadian melatonin rhythm to resetting by short wavelength light.The Journal of clinical endocrinology & metabolism, 2003. 88(9): p. 4502-4505.
- Thapan, K., J. Arendt, and D.J. Skene, An action spectrum for melatonin suppression: evidence for a novel non-rod, non-cone photoreceptor system in humans.The Journal of physiology, 2001. 535(1): p. 261-267.
- Lewy, A.J., et al., Light suppresses melatonin secretion in humans.Science, 1980. 210(4475): p. 1267-1269.
- Warman, V.L., et al., Phase advancing human circadian rhythms with short wavelength light.Neuroscience letters, 2003. 342(1-2): p. 37-40.
- Ruger, M., et al., Time-of-day-dependent effects of bright light exposure on human psychophysiology: comparison of daytime and nighttime exposure.American Journal of Physiology-regulatory, integrative and comparative physiology, 2006. 290(5): p. R1413-R1420.
- Khalsa, S.B.S., et al., A phase response curve to single brightlight pulses in human subjects.The Journal of physiology, 2003. 549(3): p. 945-952.
- Chang, A.M., et al., Human responses to bright light of different durations.The Journal of physiology, 2012. 590(13): p. 3103-3112.
- Chang, A.M., F.A. Scheer, and C.A. Czeisler, The human circadian system adapts to prior photic history.The Journal of physiology, 2011. 589(5): p. 1095-1102.
- Morgenthaler, T.I., et al., Practice parameters for the clinical evaluation and treatment of circadian rhythm sleep disorders.Sleep, 2007. 30(11): p. 1445-1459.
- Chesson, A.L., et al., Practice parameters for the use of light therapy in the treatment of sleep disorders.Sleep, 1999. 22(5): p. 641-660.
- Van Maanen, A., et al., The effects of light therapy on sleep problems: a systematic review and meta-analysis.Sleep medicine reviews, 2016. 29: p. 52-62.
- Lack, L.C. and H.R. Wright, Treating chronobiological components of chronic insomnia.Sleep Medicine, 2007. 8(6): p. 637-644.
- Gradisar, M., G. Gardner, and H. Dohnt, Recent worldwide sleep patterns and problems during adolescence: a review and meta-analysis of age, region, and sleep.Sleep medicine, 2011. 12(2): p. 110-118.
- Randler, C., C. Faßl, and N. Kalb, From Lark to Owl:developmental changes in morningness-eveningness from new-borns to early adulthood.Scientific reports, 2017. 7: p. 45874.
- Roenneberg, T., et al., A marker for the end of adolescence.Current Biology, 2004. 14(24): p. R1038-R1039.
- Chang, A.-M., etal., Evening use of light-emitting eReaders negatively affects sleep, circadian timing, and next-morning alertness.Proceedings of the National Academy of Sciences, 2015. 112(4): p. 1232-1237.
- Díaz-Morales, J.F. and C. Escribano, Social jetlag, academic achievement and cognitive performance: Understanding gender/sex differences.Chronobiology international, 2015. 32(6): p. 822-831.
- Richardson, C., et al., A randomised controlled trial of bright light therapy and morning activity for adolescents andyoung adults with Delayed Sleep-Wake Phase Disorder.Sleep Medicine, 2018. 45: p. 114-123.
- Gradisar, M., et al., A randomized controlled trial of cognitive-behavior therapy plus bright light therapy for adolescent delayed sleep phase disorder.Sleep, 2011. 34(12): p. 1671-1680.
- Skeldon, A.C., A.J. Phillips, and D.-J. Dijk, The effects of self-selected light-dark cycles and social constraints on human sleep and circadian timing: a modeling approach.Scientific reports, 2017. 7: p. 45158.
- Wright, H.R. and L.C. Lack, Effect of light wavelength on suppression and phase delay of the melatonin rhythm.Chronobiology international, 2001. 18(5): p. 801-808.
- Rahman, S.A., M.A.S. Hilaire, and S.W. Lockley, The effects of spectral tuning of evening ambient light on melatonin suppression, alertness and sleep.Physiology & behavior, 2017. 177: p. 221-229.
- Lack, L. and H. Wright, The effect of evening bright light in delaying the circadian rhythms and lengthening the sleep of early morning awakening insomniacs.sleep, 1993. 16(5): p. 436-443.
- Chellappa, S.L., et al., Non-visual effects of light on melatonin, alertness and cognitive performance: can blue-enriched light keep us alert?PloS one, 2011. 6(1): p. e16429.
- Phipps-Nelson, J., et al., Daytime exposure to bright light, as compared to dim light, decreases sleepiness and improves psychomotor vigilance performance.Sleep, 2003. 26(6): p. 695-700.
- Viola, A.U., et al., Blue-enriched white light in the workplace improves self-reported alertness, performance and sleep quality.Scandinavian journal of work, environment & health, 2008: p. 297-306.
- Partonen, T. and J. Lönnqvist, Bright light improves vitality and alleviates distress in healthy people.Journal of Affective disorders, 2000. 57(1-3): p. 55-61.
- Golden, R.N., et al., The efficacy of light therapy in the treatment of mood disorders: a review and meta-analysis of the evidence.American Journal of Psychiatry, 2005. 162(4): p. 656-662.
- Meesters, Y., et al., Low-intensity blue-enriched white light (750 lux) and standard bright light (10 000 lux) are equally effective in treating SAD. A randomized controlled study.BMC psychiatry, 2011. 11(1): p. 17.