Number of days required to reliably measure weekday and weekend total sleeping time with accelerometer: A secondary data analysis with National Health and Nutritional Survey (NHANES) 2011-2014 data.

The current standard practice for measuring sleeping time with accelerometer is to ask the participants to wear it for 7 consecutive days and analysing data from participants who have provided at least 4 days of valid data. However, this standard lacks supporting evidence. This study aims to evaluate this standard of practice by examining the reliability of measuring total sleeping time in a representative sample of US adults using accelerometer data from the National Health and Nutritional Survey (NHANES) waves 2011-2012 and 2013-2014. The sample included a total of 14,676 participants, out of which only those who provided data for seven days (n = 9510) were included in the analysis. The results revealed that the intra-class correlation coefficient (ICC) for a single day of measurement was 0.38 for weekdays and 0.27 for weekends. To achieve a reliability of 0.7, measurements for 4 and 7 nights were necessary for weekdays and weekends, respectively. Our simulation study found that the randomly-selected 3-day average of weekday sleeping time strongly correlated with the actual mean (ρ = 0.92), capturing at least 80 % of the variance. However, the randomly-selected 1-day average of weekend sleeping time only captured about 60 % of the variance. In conclusion, we recommend that future accelerometer research adopts a 9-day continuous measurement period, covering four weekend days, to reliably estimate both weekday and weekend sleeping time.