The folk belief that we should sleep 8h seems to be incorrect. Numerous studies have shown that self-reported sleep longer than 7.5h or shorter than 6.5h predicts increased mortality risk. This study examined if prospectively-determined objective sleep duration, as estimated by wrist actigraphy, was associated with mortality risks.From 1995–1999, women averaging 67.6years of age provided one-week actigraphic recordings. Survival could be estimated from follow-up continuing until 2009 for 444 of the women, with an average of 10.5years before censoring. Multivariate age-stratified Cox regression models were controlled for history of hypertension, diabetes, myocardial infarction, cancer, and major depression.Adjusted survival functions estimated 61% survival (54–69%, 95% C.I.) for those with sleep less than 300min and 78% survival (73–85%, 95% C.I.) for those with actigraphic sleep longer than 390min, as compared with survival of 90% (85–94%, 95% C.I.) for those with sleep of 300–390min. Time-in-bed, sleep efficiency and the timing of melatonin metabolite excretion were also significant mortality risk factors.This study confirms a U-shaped relationship between survival and actigraphically measured sleep durations, with the optimal objective sleep duration being shorter than the self-report optimums. People who sleep five or six hours may be reassured. Further studies are needed to identify any modifiable factors for this mortality and possible approaches to prevention.