Objective changes in activity levels following sleep extension as measured by wrist actigraphy

Objective/background: It is widely established that insufficient sleep can lead to adverse health outcomes. Paradoxically, epidemiologic research suggests that individuals who report habitual nightly sleep greater than 9 h also are at risk for adverse health outcomes. Further, studies have shown that long sleepers have decreased activity levels, which may partially explain the relationship between long sleep duration and mortality. The influence of sleep extension (longer time in bed) on levels of daily activity has not yet been established. The current study examined whether a week of sleep extension altered activity levels within the subsequent daily waking active and sleep period in order to determine whether increased time in bed indeed is related to decreased activity levels.

Methods: A total of 26 healthy volunteers wore wrist accelerometer devices (Actiwatch 2.0, Philips) in order to objectively measure sleep and activity for six days during their normal schedules and for six days during a sleep extension (10 h time in bed) intervention.

Results: There were no significant or clinically-relevant differences in 24-h activity or activity during the active or sleep period between baseline and sleep extension conditions. There were no main or interaction effects of day and condition when daily activity counts were compared between baseline and sleep extension conditions for the 24 h period (Day: F_{(5, 21)} = 1.92, p = 0.12; Condition: F_(1,25) = 2.93, p = 0.09; Day by Condition: F_(5,21) = 0.32, p = 0.83), Active Waking Period (Day: F_(5,25) = 1.53, p = 0.18; Condition: F_(1,25) = 0.26, p = 0.61; Day by Condition: F_(5,21) = 0.55, p = 0.74) or Nightly Sleep (Day: F_(5,21) = 0.86, p = 0.51; Condition: F_(1,25) = 1.78, p = 0.19; Day by Condition: F_(5,21) = 0.79, p = 0.56) periods. In contrast, there was a main effect of condition when examining sleep duration by day between conditions (Day: F_(5,21) = 1.60, p = 0.16; Condition: F_(1,25) = 167.31, p < 0.001; Day by Condition: F_(5,21) = 2.31, p = 0.07), such that sleep duration was longer during the sleep extension condition.

Discussion: Sleep duration increased during six days of a sleep extension protocol but activity levels remained similar to their baseline (normal) sleep schedule. The current findings suggest that extending time in bed alone does not alter waking activity counts in young healthy adults. The link between extended sleep and adverse health outcomes may be attributable to other phenotypic factors, or other biological correlates of extended sleep and poor health.