From the Department of Hygiene and Public Health, Nippon Medical School.
Correspondence to: Tomoyuki Kawada, MD, Department of Hygiene and Public Health, Nippon Medical School, 1-1-5 Sendagi, Bukyo-Ku, Tokyo 113-8602, Japan; e-mail: email@example.com
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I read with interest the article by Toyama et al1 in a recent issue of CHEST (March 2013). They conducted a cross-sectional study to examine the relationship between sleep indicators and lipid profiles in 275 working men. OSA and sleep duration were judged by specific devices. Among them, I have some concerns about the device Actiwatch (Philips Respironics) and its ability to determine sleep duration.
First, the authors mentioned in the Materials and Methods section that sleep duration was estimated from analysis of actigraphy tracings in conjunction with a sleep diary. Unfortunately, they did not describe procedures, and they cited only one reference for the use of actigraphy to estimate sleep-wake time.2 There is a review on the usefulness of actigraphy,3 but systemic discrepancies between actigraphy and EEG-defined sleep also exist. For example, subjects generally become inactive in a sleep-induction period, which would relate to the underestimation of sleep-onset latency.4
Second, there are several commercially based actigraphs and different scoring methods that do not have the same performance for the detection of sleep-wake judgment.5 I have a query on the information of validation for Actiwatch, which was used by Toyama et al.1 Actigraphy is an accelerometer and it does not reflect sleep itself. The cutoff value of sensitivity for making sleep/wake judgment by Actiwatch was initially set at 40 Hz. If possible, the cutoff value should be determined in each test situation by using sleep polysomnography as a gold standard.
The authors described mean (SD) of sleep duration in their Table 1 as 6.12 (0.72) and 5.93 (0.86) in subjects without dyslipidemia and with dyslipidemia, respectively. They also categorized subjects into four groups according to sleep duration in their Table 2. Furthermore, sleep duration was also used for a stepwise linear regression model to predict several lipid profiles. I think that the stable estimates of sleep duration are important for their study, and I strongly recommend that the authors describe precisely the procedure of actigraphy tracings in conjunction with sleep diary.
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