Reduced sensitivity to short-wavelength (blue) light with age has been shown for light-induced melatonin suppression. The current research aimed to determine if a similar age-related reduction occurs in subjective alertness, mood, and circadian phase-advancing responses. Young (n = 11, 23.0 +/- 2.9 years) and older (n = 15, 65.8 +/- 5.0 years) healthy males participated in laboratory sessions that included a 2-h intermittent monochromatic light exposure, individually timed to begin 8.5 h after their dim light melatonin onset (DLMO) determined in a prior visit. In separate sessions, pupil-dilated subjects were exposed to short-wavelength blue (lambda max 456 nm) and medium-wavelength green (lambda max 548 nm) light matched for photon density (6 x 1013 photons/cm2/sec). Subjective alertness, sleepiness, and mood were verbally assessed every 15 to 30 min before, during, and up to 5 h after the light exposure. The magnitude of phase advance was assessed as the difference in plasma melatonin rhythm phase markers before and after light exposure. Following blue light exposure, responses in older men were significantly diminished compared with young men for subjective alertness (p < 0.0001), sleepiness (p < 0.0001), and mood (p < 0.05) during and after light exposure. There was no significant effect of age on these parameters following green light exposure. The phase advances to both blue and green light were larger in the young than older subjects, but did not reach statistical significance. In general, phase advances to blue light were slightly larger than to green light in both young and old, but did not reach statistical significance. The current results add to previous findings demonstrating reduced responsiveness to the acute effects of blue light in older people (melatonin suppression, alertness). However, under the study paradigm, the phase-advancing response to light does not appear to be significantly impaired with age.