Sleep in healthy seniors: a diary study of the relation between bedtime and the amount of sleep obtained
The aim of this study was to determine whether naturally occurring inter-individual and intra-individual differences in bedtime selection in the elderly might be lawfully related to the amount of sleep that is obtained. A total of 128 seniors (63f, 65m) aged 70–92 years each provided a week of sleep diary data yielding a total of 896 subject-nights for analysis. From each subject-night the diary was used to derive measures of time in bed (TIB) and total sleep time (TST). These measures were used as dependent variables in mixed-effect linear models (nights nested within subjects) with the independent variable being bedtime for that subject-night, arbitrarily expressed as minutes since 19:00 hours. Although there were strong inter-individual and intra-individual differences, for both genders, bedtime had a statistically significant effect (P < 0.001) on both TIB and TST. We observed that later bedtimes were associated with less time in bed and less time asleep. On average between 7 and 8 min of less TIB and TST were associated with each 10-min delay in bedtime from 19:00 hours. These results are interpreted in terms of increases in sleep being derived from living in a better harmony with an earlier peaking circadian pacemaker characteristic of older age, although other possible mechanisms are also considered (e.g. age-dependent alterations in phase angle and homeostatic sleep need).