Apples to oranges: comparing long sleep to short sleep

Dear SirA recent paper by Zielinski et al. (2008) presents resultsfrom a study designed to assess the eﬀect of bedtime restrictionon glucose tolerance among older long sleepers. We are writingbecause we are concerned that some of the interpretations andassertions included in this paper may be misleading.The impetus for this research comes from the fact that manyepidemiological studies have observed U-shaped associationsbetween sleep duration and morbidity or mortality. Thus far,most studies have focused upon the left side of this association,i.e. associations with short sleep duration, which were consis-tent with the ﬁndings of several experimental studies of sleeprestriction in normal sleepers. Zielinski et al. have shifted ourattention to the other side of the U-shaped curve by focusingupon long sleepers. This is an important and understudied areaof research. We believe that it is mistaken, however, to assumethat the two ends of the curve reﬂect similar mechanisms orassociations.The authors state that: These data do not coincide withthose from other experimental studies that have foundimpairment of glucose⁄insulin regulation following moreextreme sleep loss (Zielinski et al., 2008; p. 416). Why wouldwe expect the results to coincide? Comparing the ﬁndings ofthe Zielinski study with those of studies of sleep restriction innormal sleepers to less than 5 or 6 h is inappropriate forseveral reasons. First, in the Zielinski study, the meanobjectively documented sleep duration after sleep restrictionwas still more than 6.5 h, which is not considered sleep loss,particularly in older individuals. So far, there is no evidencefrom either epidemiological or experimental studies to indicatethat sleep durations between 6 and 7 h may increase the risk ofimpaired glucose tolerance relative to sleep durations between7 and 8 h. Second, the assertion that sleep restriction did notimpair glucose tolerance in this study is problematic becausethe sleep-restricted group averaged only 16 min less sleep thanthe control group. Thus, the eﬀect of bedtime restriction in thisstudy had only minimal eﬀects on actual sleep duration andwas therefore unlikely to reveal a diﬀerence between thegroups on any outcome measure. Third, we would not expectto see an eﬀect on fasting glucose and insulin levels in thisstudy where actual sleep loss was very modest, because noeﬀects on fasting levels were observed in a study that involvedthe more severe bedtime restriction of 4 h in bed (Spiegel et al.,1999; see Fig. 2 within). Indeed, in this sleep debt study thatZielinski et al. cited as not coinciding with their ﬁndings, thereduction of actual sleep duration by approximately 3 h(baseline: 6 h 52 min versus debt: 3 h 48 min) per night didnot aﬀect the fasting levels, but rather the glucose and insulinresponses to a challenge (high carbohydrate meal and intra-venous glucose administration). Fourth, comparisons betweenthe previous well-controlled studies in young lean volunteersand this ﬁeld study in older adults, more than half of whomwere overweight or obese, are likely to yield disparate results.Older volunteers, particularly those who are overweight, willbegin the study with lower insulin sensitivity, and therefore amuch more sensitive method than the QUICKI on fasting and2-h postoral glucose levels used by Zielinski et al. will berequired to detect any change.The most intriguing ﬁnding of this study is the fact that theself-reported long sleepers were not, in fact, long sleepers.They averaged only 7 h of sleep out of 9 h in bed, whichindicates that these individuals have low sleep eﬃciency.Screening procedures described in the Zielinski et al. paperdo not include polysomnographic evaluation of sleep-disor-dered breathing, a condition that aﬀects a majority of olderoverweight adults and is associated with reduced sleepeﬃciency. The results of the Zielinski et al. paper suggestthat the self-reported long sleepers in the previously pub-lished studies reporting an association between long sleep andmorbidity or mortality may also have had low sleep eﬃciencyand not actually obtain more than 8 h of sleep. The ﬁndingsare important because they suggest that long self-reportedsleep may reﬂect poor sleep quality. Poor sleep quality may,in turn, be a manifestation of undiagnosed disease or itself arisk factor for disease. The association between self-reportedlong sleep and health continues to be an important area ofresearch.Kristen L. Knutson and Rachel LeproultDepartment of Medicine, University of Chicago, Chicago, IL,USA