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Nonlinear components of age-related change in sleep initiation.

BACKGROUND
Although primary studies suggest that ability to initiate sleep declines as people age, no systematic literature review has addressed the age(s) at which adults experience the greatest change in their ability to initiate sleep.


OBJECTIVE
To explore whether there are any points in time across the adult life span when the rate of change in ability to initiate sleep increases or decreases.


METHODS
Mathematical modeling was used to generate data points from information about central tendency, variance, and correlations between age and time to sleep onset provided by seven research reports. The reports represent 258 subjects ages 17 to 91 years. Smoothing splines were used to identify inflection points suggestive of major changes in sleep initiation across the life span.


RESULTS
Two mathematical models were generated. One model suggested that inflection points may exist around ages 30 and 50 years, respectively. With this model, the amount of time until sleep onset increased until the age of 30 years, but was unchanged from ages 30 to 50 years. Ability to initiate sleep appeared to decline steadily after the age of 50 years. The second model, with a p value of 0.05, lacked adequate power to identify a significant nonlinear trend.


CONCLUSIONS
Decline in ability to initiate sleep may not occur at a steady rate over the adult life span. Further research is needed to pinpoint thresholds of change and possible gender differences in thresholds.