STUDY OBJECTIVES: Evidence from cross-sectional studies shows that sleep is associated with cognitive function. This study examines change in sleep duration as a determinant of cognitive function. DESIGN: Prospective cohort. SETTING: The Whitehall II study. PARTICIPANTS: 1459 women and 3972 men aged 45-69 at baseline. INTERVENTIONS: None. MEASUREMENTS AND RESULTS: Sleep duration (≤ 5, 6, 7, 8, ≥ 9 h on an average week night) was assessed once between 1997-1999, baseline for the present study, and once between 2002-2004, average follow-up 5.4 years. Cognitive function was measured (2002-2004) using 6 tests: verbal memory, inductive reasoning (Alice Heim 4-I), verbal meaning (Mill Hill), phonemic and semantic fluency, and the Mini Mental State Examination (MMSE). In analyses adjusted for age, sex, and education, and corrected for multiple testing, adverse changes in sleep between baseline and follow-up (decrease from 6, 7, or 8 h, increase from 7 or 8 h) were associated with lower scores on most cognitive function tests. Exceptions were memory, and, for a decrease from 6-8 h only, phonemic fluency. Further adjustment for occupational position attenuated the associations slightly. However, firm evidence remained for an association between an increase from 7 or 8 h sleep and lower cognitive function for all tests, except memory, and between a decrease from 6-8 h sleep and poorer reasoning, vocabulary, and the MMSE. The magnitude of these effects was equivalent to a 4-7 year increase in age. CONCLUSIONS: These results suggest that adverse changes in sleep duration are associated with poorer cognitive function in the middle-aged. CITATION: Ferrie JE; Shipley MJ; Akbaraly TN; Marmot MG; Kivimäki M; Singh-Manoux A. Change in sleep duration and cognitive function: findings from the Whitehall II study. SLEEP 2011;34(5):565-573.