In this large population of older adults in the United States, we demonstrated that, in addition to strong associations for established and recent cases, longer daytime napping was also associated with future Parkinson disease occurrence. Compared with nonnappers, participants who napped ≥1 hour per day in 1996–1997 had an approximately 50% higher chance of reporting a Parkinson disease diagnosis in 2000 and after. This association with prediagnostic cases could not be explained by Parkinson disease medications or by inadequate nighttime sleeping and deteriorating health status, as shown in our sensitivity analyses. In contrast to napping, although atypical sleeping durations were more common among established Parkinson disease cases, they showed no relation with future Parkinson disease diagnosis.
It is well known that most Parkinson disease patients experience sleep abnormalities, ranging from difficulty in sleep maintenance, to sleep fragmentation, to conditions such as excessive daytime sleepiness and rapid eye movement sleep behavior disorder (5
). This disorder is rare in the general population but affects up to 27% of Parkinson disease patients over time (13
) and has been suggested to precede the clinical onset of Parkinson disease (13
). Not surprisingly, the current study supports the association of atypical sleep habits with Parkinson disease. As expected, the strongest associations were found for established Parkinson disease cases, which could be partially explained by their poor health status.
Excessive daytime sleepiness is the well-documented sleep disturbance that affects 16%–50% of Parkinson disease patients (18
). Longitudinal data among Parkinson disease patients show that the prevalence of excessive daytime sleepiness increases as the disease progresses, and Parkinson disease medications such as dopamine agonists may, at least partially, contribute to this problem (18
). However, most studies did not clarify whether the presence of excessive daytime sleepiness is secondary to inadequate sleep at night and/or side effects of Parkinson disease treatments, or whether it actually develops prior to Parkinson disease diagnosis. To our knowledge, only one prospective study has investigated whether daytime sleepiness appears before Parkinson disease diagnosis (2
). The analysis was conducted in the Honolulu Asia Aging Study with 43 incident cases and 7 years of follow-up of 3,078 Japanese-American men. The study showed that men who felt sleepy most of the day were about 3 times more likely to develop Parkinson disease than men who did not. Another study found an association for nurses who slept longer hours in a 24-hour period, compared with shorter sleepers, with higher future Parkinson disease occurrence (19
). However, this study did not differentiate between daytime and nighttime sleeping and might have been affected by the fact that nurses had altered sleep habits due to shift-work schedules.
The current study is substantially larger than the previous ones, included both men and women, and simultaneously examined daytime napping and nighttime sleeping. The results of our study show that longer daytime napping, but not atypical nighttime sleeping durations, is associated with future Parkinson disease occurrence. Furthermore, our sensitivity analyses of participants with a typical night sleeping duration (7–8 hours) suggest that the association of longer day napping and future Parkinson disease occurrence could not be explained simply by short night sleeping.
The lack of an association between nighttime sleeping and future Parkinson disease occurrence may need to be approached cautiously. Although these data may suggest that indeed nighttime sleeping is not disturbed during the premotor stage of Parkinson disease, this finding may be due to measurement errors regarding nighttime sleeping duration. Self-reported night sleeping duration is both approximate and subjective, and it may not reflect the quality of nighttime sleeping and the various sleep cycles. Objective, yet relatively expensive polysonographic assessment provides much better data on the quality and quantity of nighttime sleeping but is not suitable for use in large populations. Because daytime napping generally involves less than one sleep cycle, people tend to arise if they awake from a daytime nap; thus, reports of daytime napping time may be more accurate.
Although this study has several strengths, as mentioned earlier, it also has some limitations. Daytime napping is only a surrogate for excessive daytime sleepiness, because whether or not one can actually nap depends on several factors in addition to his or her tendency to sleep. Therefore, our result may not be directly comparable to those from Abbott et al. (2
), who aimed to directly ask about daytime sleepiness. In the current study, we adjusted for and stratified by some known Parkinson disease risk factors and conducted several sensitivity analyses to examine the robustness of our findings; however, we did not have data on some other potential confounders such as head injury that may be associated with both sleep duration and Parkinson disease (20
). Therefore, we were unable to exclude the possibility of unmeasured confounding.
We relied on self-reports for case identification because it was necessary in such a large population-based cohort. It is likely that some cases were not identified and some noncases were misclassified as cases. On the other hand, the ongoing validation study suggests that most self-reports can be confirmed by the treating neurologists or by expert medical record review. Furthermore, we previously reported the well-known association of smoking with Parkinson disease in this cohort, which indirectly supports our case identification strategy (22
). We did not have information on the cohort's use of dopaminergic medicines, which may alter sleeping/napping habits; therefore, we were unable to examine how the use of dopaminergic medicines might have affected our analyses for prevalent and recent cases.
As in many other epidemiologic studies (23
), hours of napping was self-reported; therefore, misclassification of exposure is also possible. For incident cases diagnosed in 2000 and after, however, these misclassifications were likely nondifferential and therefore might have underestimated the strength of the association between napping and future Parkinson disease. The analyses were limited to participants of the follow-up survey in 2004–2006, which included approximately only 66.0% of the risk factor survey participants. A selection bias could have been introduced if deaths or losses to follow-up had been differentially related to napping habits by Parkinson disease status. Finally, previous studies show rapid eye movement sleep behavior disorder may predate Parkinson disease by a decade (14
), whereas our study did not collect data on this disorder and had a shorter follow-up interval. Nonetheless, our study suggests that daytime napping, not atypical nighttime sleep duration, is associated with a higher future risk of Parkinson disease.