The parable of parabola: what the U-shaped curve can and cannot tell us about sleep.

The Finnish Twin Cohort and in Great Britain, The Whitehall II Cohort--both endowed with rich covariate data. Associations between short and long sleep durations and mortality were first noted by Hammond in 1964 in the American Cancer Society I study, and they later achieved notoriety when presented by Kripke et al in 1979 (see reference 5 for a review of studies with similar findings.) Although the number and specificity of publications to further explore these controversial findings rapidly increased, most studies relied on the association of sleep duration measured at a single time point with followed-up mortality. Noteworthy in the Hublin et al and Ferrie et al studies is that participants were queried about their sleep durations at two different time points separated by intervals of 5 to 6 years, and mortality was assessed 22 and 12 years, respectively, subsequent to the second measurement. This feature effectively eliminates the possibility that the tails of the habitual sleep distribution (short or long sleep) were reflecting terminal illnesses or death-imminent processes, even in their most subtle stages. Only one prior study attempted to account for this limitation by eliminating deaths shortly after the ascertainment of sleep duration, but the two-year interval used may not have entirely eliminated this possibility. The repeated surveys in both Hublin et al and Ferrie et al demonstrate reliability in the subjects' answers over time, indicating that these were not spurious or "random" estimates of sleep duration. Furthermore, the repeated measures allowed investigation of change in sleep duration over time. Particularly interesting in Ferrie et al was that increases in sleep duration over time (from less than 5 to 7 hours or more) in some individuals was associated