Effects of aging on the intrinsic circadian period of totally blind humans

Age-Related changes in the intrinsic circadian period (τ) have been hypothesized to account for sleep symptoms in the elderly such as early morning awakening. The authors sought to determine whether the aging process produced quantifiable differences in the τ of totally blind men who had free-Running circadian rhythms. The melatonin onset was used as the indicator of circadian phase. Melatonin rhythms had been characterized about a decade previously when the participants were 38 ± 6 (SD) years old. Both previous current assessments of τ were derived from at least 3 serial measurements of the 24-H melatonin profile from which the melatonin onset was determined. All 6 participants exhibited a longer τ in the 2nd assessment (mean increase ± SD of 0.13 ± 0.08 h; p < 0.01). Four participants exhibited differences in τ with nonoverlapping 95% confidence intervals. The results do not support the Commonly held view that shortens during human aging. On the contrary τ appears to slightly but significantly lengthen during at least 1 decade in midlife.