Figure 1.
Melatonin suppression protocol.
Experimental sessions started at 20:00 h and lasted until 04:00 h. Subjects were in dim light (<1 lux) between 20:00 h and 22:00 h and in total darkness afterwards. Pupils were fully dilated using antimuscarinic eye drops with one drop at three recurrences (−60, −45 and −30 minutes) prior light exposure. Subjects were exposed to 60 minutes (00:30 h–01:30 h) of monochromatic light at 9 different wavelengths (420–620 nm) of equal photon density (3.16×1013 photons/cm2/sec) over 9 experimental sessions. Outside light stimulation segment during which they were in a sitting position (90°) participants were in a semi recumbent position (45°). Blood samples were collected every 15–60 minutes and plasma melatonin was assayed by RIA. Control-adjusted melatonin suppression was compared across light treatments and sensitivity spectra were established for each subject.
Figure 2.
Evaluation of non-visual sensitivity to light via melatonin suppression.
Profiles of plasma melatonin in a representative older subject during 3 experimental sessions, with exposures to 2 monochromatic lights (530 (green line) and 590 nm (red line)) of equal photon density, and a control dark session. Note the wavelength-dependent acute effects of the monochromatic lights compared to control (black line). A relatively stable phase angle of entrainment is observed for this subject across this 4 weeks' study segment.
Figure 3.
Non-visual spectral sensitivity to light (melatonin suppression).
A. Melatonin suppression in young (n = 5) and (B) older participants (n = 8) is represented in percentage control adjusted (%CA). In both young and older groups short (420 nm) and long wavelengths lights (>560 nm) have a limited effect on melatonin suppression. Using a four parameter Gaussian fitting procedure, peak sensitivity is found at 484 nm in the young (blue fit curve, R2 = 0.94 and p<0.01 on raw values; R2 = 0.81 and p<0.05 on log-transformed values)and 494 nm in the older participant (red fit curve, R2 = 0.90 and p<0.01 on raw values; R2 = 0.89 and p<0.01 on log-transformed values). C. Comparison of spectral sensitivity in young (blue fit curve) and older subjects (red fit curve). Older subjects show an altered spectral sensitivity of melatonin suppression compared to young subjects. Sensitivity to light in the older is similar in the short wavelength region of the spectrum (<500 nm), but higher for 500–590 nm, and with a shift of peak sensitivity from 484 nm (young) to 494 nm (older).
Figure 4.
Changes in lens transmittance with aging and predicted impact on non-visual spectral sensitivity to light.
A. Transmittance spectra of the ocular lens in the young (n = 8, red line), and older subjects (n = 8, blue line). Filtering is particularly pronounced in the short to medium wavelength range (<530 nm) in the old compared to young subjects. Symbols on the curves indicate the average lens transmittance at 480 nm in the young (blue circle, 74.4%) and in the old subjects (red circle, 42.9%). An average decreased transmittance by 42.3% is found between young and old. B. real melatonin suppression spectrum in young subjects (blue line), and predicted melatonin suppression spectrum in the older subjects based on their decreased lens transmittance (red dashed line). Note the expected decreased melatonin suppression to short wavelength lights in the elderly and the shift in peak sensitivity to light from 484 nm to 497 nm. A 34% attenuation in melatonin suppression is predicted at 480 nm (see Figure S2 and File S1 for details).
Figure 5.
Melatonin suppression and lens transmittance at 480 nm light, in young and aged subjects.
Lens transmittance (open circle, straight line) is significantly lower in older compared to young subjects (p<0.0001) whereas melatonin suppression (full circle, straight line) at 480 nm is not significantly different between aged and young. The predicted effect of a decrease in lens transmittance at 480 nm on melatonin suppression is shown in gray circle, dashed line. Prediction was made using Brainard et al. 2001 irradiance response curves. See File S1 for more details on the analytics of the prediction.