COMMENTARY
To Nap or Not to Nap: That is the Question
http://dx.doi.org/10.5665/sleep.1946
Commentary on Cohen-Mansfield et al. Sleep duration, nap habits, and mortality in older persons. SLEEP 2012;35:1003-1009. Reena Mehra, MD, MS1; Sanja R. Patel, MD, MS2 1
Case Western Reserve University, Cleveland, OH; 2Brigham and Women’s Hospital, Boston, MA
Both short and long sleep duration have been shown to be associated with adverse health outcomes and increased mortality, with a fairly consistently reported U-shaped distribution and increased risks at both ends of self-reported sleep time. The effect of napping on health outcomes and mortality, however, is a bit more inconsistent and ambiguous. In this issue of SLEEP, Cohen-Mansfield and colleagues1 studied night-time duration and nap habits in an Israeli cohort of 1,644 older individuals 75-94 years of age, the Cross-Sectional and Longitudinal Aging Study (CALAS), for which 20-year follow-up mortality data was available. The authors sought to examine the effect of self-reported nighttime sleep duration on mortality and the effect modification of daytime napping on the relationship of sleep duration and mortality. The proposed hypothesis was to observe an increase in mortality for long sleepers (> 9 hours per night) and short sleepers (< 7 hours per night). The authors concluded that long sleep was associated with an approximately 30% increased risk of mortality compared to those obtaining 7-9 hours of sleep after adjusting for confounders including subject characteristics and comorbid factors. However, short sleep was not associated with an increase in mortality in the fully adjusted model. In those who reported napping, a survival benefit was noted in those obtaining < 7 hours of sleep, translating into an approximate 20% improvement in mortality. In contrast, a deleterious effect of napping was noted in those obtaining > 9 hours of sleep, consistent with an approximate 40% increase in mortality compared to those individuals obtaining “normal” duration sleep (i.e., 7-9 hours). Furthermore, although the interaction of sleep duration and napping was statistically significant in the unadjusted and partially adjusted models, the interaction term was not significant in the fully adjusted model taking into consideration confounding by health status variables, including comorbidity, medications, and depression. The strengths of the study include the large sample size and relatively novel examination of the interplay of napping in sleep duration and mortality. Study limitations include the self-report and subjective nature of obtaining the sleep duration and nap habit measures, restricted generalizability, and the suboptimal fidelity of these measures assessed at one point in time as reflective of baseline sleep characteristics predicting future mortality over the 20-year follow-up period.
This study complements contemporary studies in regard to potential mechanisms affecting population health. In this cohort with mean age of 83 years, long but not short sleep duration was associated with increased mortality. Existing data, including those derived from an older cohort, suggest that long sleep duration may be associated with up-regulation of pro-inflammatory pathways involving increases in hs-C-reactive protein, interleukin-6, and fibrinogen.2,3 These data suggest a role for increased systemic inflammation in long-sleep duration pathogenesis, including the somnogenic effect of cytokines, which may exert detriment in a bi-directional manner relative to long sleep duration to perpetuate the process of increasing adverse outcomes and mortality. Although the long sleep duration-mortality relationship in the current study was noted independent of comorbidities including depression, the possibility of residual confounding cannot be excluded, particularly as many of the comorbidities may increase risk of fatigue, sleepiness, and a bedbound state and potential confounding from sleep disorders. The absence of short sleep duration as a mortality risk in this cohort is somewhat unusual, particularly in the face of a wealth of data that has amassed implicating short sleep duration as a contributor to increased mortality. The current findings could be a result of measurement error of sleep duration given it is well recognized that self-reported sleep duration is often an overestimate of true sleep duration. The survivorship bias could also exist, such that those individuals who have survived may be more immune to the negative sequelae of short sleep duration, or resistant because of a background effects (e.g., genetic susceptibilities). Furthermore, the differences in the relationship of short versus long sleep and mortality may be due to the notion that sleep duration represents a surrogate for social interactions, which may be the true causal factor. The highlight feature in this study by Cohen-Mansfield et al.,1 however, is the observed effect of napping on the relationship of sleep duration and mortality in the stratification analysis. Why would short nighttime sleepers (< 7 hours) experience a reduced mortality (i.e., a protective effect), in those with self-reported napping? One possibility is that theoretically the total sleep time achieved over a 24-hour period may come closer to the optimal 8 hours recommended for the average adult in those who have short sleep along with the additional sleep time supplemented by napping. However, following this logic, it is still unclear why this sleep pattern would convey a benefit of reduced mortality compared to those individuals who report “normal” sleep duration of 7-8 hours. A potential basis could be a protective effect of siesta on mortality,4 which has been described with relative inconsistency in the literature. On the other side of the sleep duration spectrum, however, the opposite finding was noted with long sleep duration such that an
Submitted for publication May, 2012 Accepted for publication May, 2012 Address correspondence to: Reena Mehra, MD, FAASM, Case Western Reserve University, 11100 Euclid Avenue, Cleveland Heights, OH 44106; Tel: (216) 844-5128; Fax: (216) 844-8708; E-mail:
[email protected] SLEEP, Vol. 35, No. 7, 2012
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CITATION Mehra R; Patel SR. To nap or not to nap: that is the question. SLEEP 2012;35(7):903-904.
increase in mortality was observed. Recent data from an older cohort of men also support an association between long sleep duration and napping.5 Conceivably, in those with self-reported napping, a further increase in sleep in those designated as long sleepers may result in an augmented increase in mortality due to enhancement of a pro-inflammatory state, although existing data do not specifically describe graded relationships of increasing sleep time with increasing levels of systemic inflammation. Notably, the mitigation of the napping statistical interaction in the fully adjusted model likely reflects either an underpowered sample size or true confounding by poorer health. Although the effect modification by napping is intriguing, the possibility that this finding represents a false positive should be considered. Aspects that refute this include the fact that the magnitude of the difference is clinically important, the subgroup analysis was one of a small number tested, and there appears to be indirect evidence supporting the difference. The current work highlights the importance of examining the intersection of sleep duration, sleep-wake patterning and wake time napping and mortality and also uncovers complexities in data interpretation when investigating the relationships of sleep duration and day/night sleep-wake patterns on outcomes. Future epidemiologic studies should be conducted to examine the relationships of sleep duration, napping, and health outcomes with additional attention to circadian rhythm influences to assess reproducibility of the current findings and also to unveil potential mediating pathophysiologic pathways.
SLEEP, Vol. 35, No. 7, 2012
DISCLOSURE STATEMENT Dr. Mehra serves on the Care Core medical advisory board; University Hospitals Case Medical Center has received PAP machines and other equipment for the conduct of research for which Dr. Mehra is the principal investigator. Dr. Patel serves as a consultant for Sleep HealthCenters and Apnex and has received research support from Philips Respironics. REFERENCES
1. Cohen-Mansfield J, Perach R. Sleep duration, nap habits, and mortality in older persons. Sleep 2012;35:1003-9. 2. Dowd JB, Goldman N, Weinstein M. Sleep duration, sleep quality, and biomarkers of inflammation in a Taiwanese population. Ann Epidemiol 2011;21:799-806. 3. Patel SR, Zhu X, Storfer-Isser A, Mehra R, Jenny NS, Tracy R, Redline S. Sleep duration and biomarkers of inflammation. Sleep 2009;32:200-4. 4. Naska A, Oikonomou E, Trichopoulou TP, Trichopoulos D. Siesta in healthy adults and coronary mortality in the general population. Arch Intern Med 2007;167:296-301. 5. Patel SR, Blackwell T, Ancoli-Israel S, Stone KL; for the Osteoporotic Fractures in Men (MrOS) Research Group. Sleep characteristics of selfreported long sleepers. Sleep 2012;35:641-8.
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