Age-associated Advance of Sleep Times Relative to the Circadian Phase of Alertness-Sleepiness Rhythm: Can it be Explained by Changes in Ratios Between Strengths of the Underlying Oscillatory Processes?

The aging process is often associated with more or less prominent shifts of the entrained phases of behavioral and physiological rhythms at earlier clock hours, but the oscillatory mechanisms underlying these shifts have to be determined. The analysis of self-reports on home sleep times and self-scorings of sleepiness provided by 130 participants of sleep deprivation experiments revealed the difference between intro-individual (age-related) and inter-individual (gender- and chronotype-related) variation in phase angle between sleep timing and timing of nocturnal rise of sleepiness. A heterochronity of age-related changes in phase characteristics of the circadian rhythms was suggested for explaining this difference. In particular, a rhythm's strength might determine the rate of age-related decline of its entrainment mechanism. In the case of a weak circadian rhythm, such as the homeostatic process underlying oscillations of slow wave activity in the sleep-wake cycle, a decline of the rhythm's strength might be already pronounced in middle aged adults. In contrast, a similar decline might occur much later in the cases of the homeostatic processes underlying stronger rhythms, such as the fluctuations of alertness-sleepiness, body temperature, melatonin secretion, etc. Moreover, the strength of a strong rhythm might become weaker earlier than that of the circadian pacemaker and, especially, earlier than decline of its ability to entrain to the light-dark cycle. Such sequence of changes in the circadian entrainment mechanisms might explain a general tendency to produce advance rather than delay shifts of entrained phases of circadian rhythms with progression of age from middle adulthood to elderly.