The relevance of daylight for humans.

Daylight is ubiquitous and is crucial for mammalian vision as well as for non-visual input to the brain via the intrinsically photosensitive retinal ganglion cells (ipRGCs) that express the photopigment melanopsin. The ipRGCs project to the circadian clock in the suprachiasmatic nuclei and thereby ensure entrainment to the 24-hour day-night cycle, and changes in daylength trigger the appropriate seasonal behaviours. The ipRGCs also project to the perihabenular nucleus and surrounding brain regions that modulate mood, stress and learning in animals and humans. Given that light has strong direct effects on mood, cognition, alertness, performance, and sleep, light can be considered a "drug" to treat many clinical conditions. Light therapy is already well established for winter and other depressions and circadian sleep disorders. Beyond visual and non-visual effects via the retina, daylight contributes to prevent myopia in the young by its impact on eye development, and is important for Vitamin D synthesis and bone health via the skin. The sun is the most powerful light source and, dependent on dose, its ultraviolet radiance is toxic for living organisms and can be used as a disinfectant. Most research involves laboratory-based electric light, without the dynamic and spectral changes that daylight undergoes moment by moment. There is a gap between the importance of daylight for human beings and the amount of research being done on this subject. Daylight is taken for granted as an environmental factor, to be enjoyed or avoided, according to conditions. More daylight awareness in architecture and urban design beyond aesthetic values and visual comfort may lead to higher quality work and living environments. Although we do not yet have a factual basis for the assumption that natural daylight is overall "better" than electric light, the environmental debate mandates serious consideration of sunlight not just for solar power but also as biologically necessary for sustainable and healthy living.

Sleep Restriction With Circadian Disruption Negatively Alter Bone Turnover Markers in Women.

PURPOSE: The purpose of this work is to determine whether an uncoupling of bone turnover markers (BTMs) occurs in women exposed to the combination of sleep restriction with circadian disruption (SRCD), as previously reported in men. METHODS: Four bone biomarkers (N-terminal propeptide of type I procollagen [P1NP] and osteocalcin = bone formation; C-telopeptide [CTX] = bone resorption; sclerostin = bone formation inhibitor) were measured in bihourly samples over 24 hours at baseline and after approximately 3 weeks of sleep restriction (~5.6 hours of sleep/24 hours) with concurrent circadian disruption (SRCD, recurring 28-hour "day" in dim light). Maximum likelihood estimation in a repeated-measures model was used to assess the effects of SRCD and age on bone biomarkers. RESULTS: Five women were young (22 ±â€…2.8 years) and four were older (58 ±â€…1.8 years). Baseline bone biomarker levels did not differ by age (all P ≥ .07). Bone formation markers were lower after SRCD (estimate ±â€…SEE, ΔP1NP = -9.5 ±â€…2.8 µg/L, P = .01; Δosteocalcin = -2.3 ±â€…0.9 ng/mL, P = .04). The P1NP decline was greater in young women (ΔP1NP = -12.9 ±â€…3.7 µg/L, P = .01). After SRCD, CTX was significantly higher in young women (0.182 ±â€…0.069 ng/mL, P = .04) but did not change in older women. CONCLUSIONS: These pilot data are similar to previous findings in men and suggest that SRCD negatively altered bone metabolism in women by decreasing markers of bone formation and, in young women, increasing a marker of bone resorption. If sustained, this pattern of BTM uncoupling may lead to bone loss and lower bone mineral density.

Improved cognitive morning performance in healthy older adults following blue-enriched light exposure on the previous evening.

OBJECTIVES: Exposure to light can have acute alerting and circadian phase-shifting effects. This study investigated the effects of evening exposure to blue-enriched polychromatic white (BEL) vs. polychromatic white light (WL) on sleep inertia dissipation the following morning in older adults. METHODS: Ten healthy older adults (average age = 63.3 yrs; 6F) participated in a 13-day study comprising three baseline days, an initial circadian phase assessment, four days with 2-h evening light exposures, a post light exposure circadian phase assessment and three recovery days. Participants were randomized to either BEL or WL of the same irradiance for the four evening light exposures. On the next mornings at 2, 12, 22 and 32 min after each wake time, the participants completed a 90-s digit-symbol substitution test (DSST) to assess working memory, and objective alertness was assessed using a wake EEG recording. DSST and power density from the wake EEG recordings were compared between the two groups. RESULTS: DSST performance improved with time awake (p < 0.0001) and across study days in both light exposure groups (p < 0.0001). There was no main effect of group, although we observed a significant day x group interaction (p = 0.0004), whereby participants exposed to BEL performed significantly better on the first two mornings after light exposures than participants in WL (post-hoc, p < 0.05). On those days, the BEL group showed higher EEG activity in some of the frequency bins in the sigma and beta range (p < 0.05) on the wake EEG. CONCLUSION: Exposure to blue-enriched white light in the evening significantly improved DSST performance the following morning when compared to polychromatic white light. This was associated with a higher level of objective alertness on the wake EEG, but not with changes in sleep or circadian timing.

Light and chronobiology: implications for health and disease.

Environmental light synchronizes the primary mammalian biological clock in the suprachiasmatic nuclei, as well as many peripheral clocks in tissues and cells, to the solar 24-hour day. Light is the strongest synchronizing agent (zeitgeber) for the circadian system, and therefore keeps most biological and psychological rhythms internally synchronized, which is important for optimum function. Circadian sleep-wake disruptions and chronic circadian misalignment, as often observed in psychiatric and neurodegenerative illness, can be treated with light therapy. The beneficial effect on circadian synchronization, sleep quality, mood, and cognitive performance depends on timing, intensity, and spectral composition of light exposure. Tailoring and optimizing indoor lighting conditions may be an approach to improve wellbeing, alertness, and cognitive performance and, in the long term, producing health benefits.

La luz ambiental sincroniza el reloj biológico primario de los mamíferos en el núcleo supraquiasmático, así como muchos relojes periféricos en tejídos y células, para el día solar de 24 horas. La luz es el agente sincronizador más potente (zeitgeber=dador de tiempo) para el sistema circadíano, y por consiguiente mantiene la mayoría de los ritmos biológicos y psicológicos que se sincronizan internamente, lo que es importante para una óptima función. Las disrupciones circadianas sueño-vigilia y los desajustes circadianos crónicos que se observan con frecuencía en enfermedades psiquiátricas y neurodegenerativas pueden ser tratados con fototerapía. Los efectos favorables sobre la sincronización circadíana, la calidad del sueño, el ánimo y el rendimiento cognitivo dependen de la duración, intensidad y composición espectral de la exposición a la luz. La adaptación y optímízacíón de las condíciones de luz interior pueden constituir una manera de mejorar el bienestar, el alerta y el rendimiento cognitivo, y a largo plazo producir beneficios para la salud.

La lumière environnementale synchronise l'horloge biologique primaire des mammifères dans le noyau suprachiasmatique, ainsi que de nombreuses horloges périphériques dans les tissus et les cellules, au jour solaire de 24 h. La lumière est l'agent synchronisant le plus fort (zeitgeber) du système circadien, conservant donc la plupart des rythmes biologiques et psychologiques synchronisés en interne, ce qui est important pour un fonctionnement optimal. Les troubles circadiens veille-sommeil et le décalage circadien chronique, souvent observés dans les maladies psychiatriques et neurodégénératives, peuvent être traités par luminothérapie. L'effet bénéfique sur la synchronisation circadienne, la qualité du sommeil, l'humeur et la performance cognitive dépend de la chronologie, de l'intensité et de la composition spectrale de l'exposition à la lumière. Personnaliser et optimiser les conditions de lumière en intérieur pourraient permettre d'améliorer le bien-être, la vigilance et la performance cognitive et, à long terme, être bénéfique pour la santé.