Renske Lok, Ph.D. Stanford University, Department of Psychiatry and Behavioral Sciences
Beyond its role in vision, light plays a significant role in shaping our physiology and behavior. Light exposure at night influences alertness, cognitive functioning, and melatonin production. The effects of light on these non-visual functions are contingent upon factors such as intensity, timing, temporal pattern, and spectral properties of light exposure, and there is compelling evidence that these dependencies may vary with age.
Photo by Renske Lok
Aging and light intensity
Numerous studies investigating the non-image-forming effects of light intensities have been conducted in young, healthy adults (18-40 years). However, an increasing number of studies underscore that such effects may not be the same over the lifespan. For example, when investigating how evening light of different intensities (ranging from 5 to 5000 lux) relates to melatonin suppression in preschool-aged children (~3 to 5 years), researchers found that melatonin levels were suppressed across the full range of intensities examined (Hartstein et al., 2022). In contrast, in those aged 18-44, half of the fitted maximal melatonin-suppressing effect of light is estimated to be induced by ∼50 – 130 lux (Cajochen et al., 2000). While there wasn't a clear intensity-dependent response in preschool children, the lowest quartile of light intensities (5–40 lux) showed a notably lower average melatonin suppression than the higher quartiles, indicating that the chosen light intensities might have been too high for this age group. On the other side of the spectrum, older adults (~60 years and up) exhibit a lower sensitivity to light compared to their younger counterparts, suggesting that older individuals may require increased light exposure to attain the same benefits experienced by the younger population (Duffy et al., 2007). Age-related ocular conditions, such as cataracts, lens yellowing, and behavioral changes, contribute to decreased light exposure for older individuals. This population often resides in dimly lit environments and has restricted access to natural daylight, ultimately diminishing circadian light sensitivity by limiting the amount of light reaching the retina.
Aging and light spectrum
The prevailing consensus is that the circadian system exhibits its highest sensitivity to light at approximately 480 nm in wavelength. This particular wavelength maximally stimulates the melanopsin-sensitive photoreceptor, which plays a predominant role in the non-visual effects of light. While other photoreceptors, such as those sensitive to shorter wavelengths (S-cone) or longer wavelengths (M-cone or L-cone), also contribute to non-image forming effects, their impact is comparatively less significant than that of the melanopsin-sensitive light cells. A recent study proposes that the involvement of other photoreceptors in the non-image-forming effects of light may change over the lifespan (Najjar et al., 2024). In a within-subject design, researchers exposed young and older individuals to narrow-band lights ranging from 420 to 620 nm for 60 minutes at night to assess the effects on melatonin suppression. The study found a straightforward pattern in young adult participants (~25 years): melanopsin solely drove melatonin suppression at all time intervals. The peak sensitivity was identified at 485.3 nm after just 15 minutes of light exposure. However, in the older group (~59 years), the process was jointly driven by melanopsin, the short-wavelength-sensitive (S)-cone and medium-wavelength (M)-cone, with a stable peak sensitivity around 500 nm at 30, 45, and 60 minutes of light exposure. This suggests that as humans age, there's a shift from a reliance on melanopsin alone to a more intricate interplay of photoreceptors in regulating melatonin suppression.
Future perspectives
Collectively, these studies mark the initial strides toward a nuanced comprehension of the non-image-forming effects of light across diverse life stages. The shift from the reliance on melanopsin in youth to the intricate interplay of photoreceptors in older age, along with the notable impact of low-intensity evening light on children versus higher-intensity lighting in adults, opens doors for tailored approaches to light therapy. It underscores the crucial role of a well-considered lighting environment at every stage of life and emphasizes the significance of careful sample selection in research studies.
However, a need remains to unravel how a combination of different light intensity and spectral composition variations influence melatonin suppression and whether these lighting effects extend to other non-image-forming effects, such as mood and cognitive performance. Further exploration in these areas will contribute to a more comprehensive understanding of the intricate, multifaceted relationship between light and well-being across the lifespan.
References
Cajochen, C., Zeitzer, J.M., Czeisler, C.A., Dijk, D.J., 2000. Dose-response relationship for light intensity and ocular and electroencephalographic correlates of human. Behavioural Brain Research 115, 75–83.
Duffy, J.F., Zeitzer, J.M., Czeisler, C.A., 2007. Decreased sensitivity to phase-delaying effects of moderate intensity light in older subjects. Neurobiology of Aging 28, 799–807.
Hartstein, L.E., Behn, C.D., Akacem, L.D., Stack, N., Wright Jr, K.P., LeBourgeois, M.K., 2022. High sensitivity of melatonin suppression response to evening light in preschool‐aged children. Journal of pineal research 72 (2), e12780.
Najjar, R.P., Prayag, A.S., Gronfier, C., 2024. Melatonin suppression by light involves different retinal photoreceptors in young and older adults. Journal of pineal research 76 (1), e12930.