Habitual ‘short sleep’: six hours is ‘safe’

Dear Sir The recent meta-analysis and review of sleep duration and mortality in adults, by Gallicchio and Kalesan (2009, pp. 148–158) concluded that while long sleep (>9 h) is a risk for all-cause mortalities, this also applied to sleeping fewer than 7 h. However, where there are implications from various studies that habitual 6 h sleep carries a significant long-term health risk [other than excessive daytime sleepiness (EDS)], these are based mainly on generalizations from people habitually sleeping fewer than 5 h, where there may well be risks; there is little evidence to demonstrate that this conclusion applies to those sleeping 6–7 h. Furthermore, only about 5–8% of the adult population sleep habitually for 5 h or less (e.g. Groeger et al., 2004). This topical area is relevant to the issues of sleep debt being widespread in contemporary society, and the apparent need for around 8 h sleep a night. Both these latter assumptions should also be viewed against: (i) the functions of sleep still not being understood fully and (ii) the normal distribution in habitual sleep duration. Advocating the need for >6 h sleep adds to the worries of the typical insomniac patient, who usually obtains 6 h sleep and is not sleepy during the day (American Association of Sleep Medicine (AASM), 2004). I argue that 6 h habitual daily sleep, whether it be in healthy sleepers or those with insomnia, actually carries little or no extra risk of ill health or even eventual death than for those people having around 8 h sleep, unless there is EDS, which is clearly evident of inadequate sleep. I base my argument upon several issues. First, most of the prospective cohort studies reporting on short sleep, morbidity and mortality present methodological difficulties in the estimation and categorization of sleep durations: • Sleep length is based usually upon subjective responses to one question (cf. all papers covered by Gallicchio and Kalesan, 2009). Although it could be argued that the extent to which people mis-estimate their sleep contains random errors that will average out, this has not been demonstrated. • Few, if any, studies provide respondents with a definition of what is meant by sleep length ; a term confused easily with time in bed . Such data need confirmation with respect to times of: going to bed, sleep onset, morning awakening and getting up. • Weekend sleep is seldom, if ever, assessed or included in these studies. What if (which must be likely) an individual is a short sleeper by weekday but a long sleeper at weekends? • The most important difficulty relates to the response categories for sleep length and analyses. There two aspects to this problem: (a) Most studies (cf. Gallicchio and Kalesan, 2009) claiming that <7 h sleep poses risks combine all respondents sleeping fewer than 7 h and compare them with 7–8 h or 9 h sleepers. As the <7 h group contains those sleeping fewer than 5 h (where there is a risk), the same risk is assumed to apply to 6–7 h sleepers, where there is no specific evidence to support this latter notion. (b) Some studies have improved upon this, up to a point, in banding sleep in apparently 1 h intervals. For example, the large cohort study of 8354 civil servants (cf. Ferrie et al., 2007) classed sleep as: 5 h or less, 6, 7, 8 and 9 h or more ; but there is a missing hour. For example, someone with 51⁄4 h sleep (presumably riskier than 6 h) is included within the 6 h category, which probably extends up to 61⁄2 h. Although Ferrie et al. reported a relative risk (RR) of mortality (all causes) of 1.22 for the 6 h band, compared with a RR = 1 for the 7 h sleepers, the former contained those sleeping just above 5 h. Secondly, even for the few findings of a higher RR for morbidity ⁄mortality attributable to a habitual daily sleep of, for instance, 6–7 h, the RR is generally low (albeit significant because of large sample sizes), especially when seen in absolute rather than relative terms. For example, the Sleep, Heart Health Study (Gottlieb et al., 2006), involving 5910 adults, implied that 6–7 h sleepers had a significantly higher risk of hypertension compared with 7–8 h sleepers. However, the difference in mean systolic blood pressure (BP) between the groups was only 2.1 mmHg (130.5 mmHg versus 128.4 mmHg) and 0.7 mmHg for diastolic BP (75.0 mmHg versus 74.3 mmHg). Such values are well under the usual hypertensive thresholds. More to the point is that increasing sleep by an hour or so is unlikely to lower BP, whereas spending a portion of this extra hour exercising would be therapeutic. Thirdly, more recent studies, subsequent to Gallicchio and Kalesan s review, also cast doubt on these risks for 6 h habitual sleep. Stranges et al. (2008) reported on a cohort from the Western New York Health Study (1996–2001; n = 3027) from asking the single question: on the average, how many hours did you sleep each night during the last 5 weekday nights? Weekends were excluded. Data were collapsed into only three categories (note): <6 h, 6–8 h and long sleep >8 h. Poorer health was attributed only to those sleeping fewer than 6 h. Krueger and Friedman s (2009) survey of 110 441 adults utilized the single question: on average how many hours of sleep do you get in a 24 h period? Responses were to the nearest hour (5 or fewer hours, 6, 7, 8 h, etc.), with 6 h extending from just over 5 h. While there was a 10% greater prevalence (RR: 1.1) for various diseases in the 6 h grouping, J. Sleep Res. (2010) 19, 119–120 Letter to the Editor