Physical activity and sedentary behaviour were calculated using the data from the accelerometer. The accelerometer data were first downloaded by the ActiLife software (version 6.4.11) into ActiGraph counts metric (.gt3x file format) and converted into .agd file format without Low Frequency Extension Filter [20]. Thereafter, the data files were analyzed using a 60-s epoch and produced counts per minute (cpm) in vector magnitude by a custom-made software (Propero) developed at the Department of Sports Science and Clinical Biomechanics, University of Southern Denmark. Valid data was defined as those with minimum 10-h recordings for at least 4 days. Periods of 30 min or longer of continuous zero counts allowing for one single spike below 100 counts was defined as non-wear and set as missing in the analysis. TIB of each night was estimated from the self-reported accelerometer wear-time diary as the time from going to bed until getting out of bed the next day. TIB periods was set as zero in the individual accelerometry recordings and thus identified as non-wear. Finally, physical activity and sedentary behavior were summed for the accelerometer wear time between 05:00–24:00 (maximum 19 h/day) and time spent in the defined intensities of physical activity and sedentary behavior were calculated using accelerometer wear time as denominator and expressed in percentage.

Sedentary behavior was defined as 0–2302 cpm in vector magnitude, according to a validated cut-point for wrist-worn accelerometers in older adults with thigh-worn accelerometer (ActivPAL) as reference [21]. Being highly sedentary (≥10 h/day) has been linked to several adverse health indicators, including diminished physical function [22] and increased risk for cardiovascular disease [23]. In this study, highly sedentary was defined as having sedentary behavior of ≥65% of accelerometer wear time (median 15 h 25 min/day), corresponding to 10 h/day in the current study population. There are currently no available cut-points reflecting moderate and vigorous intensity physical activity for wrist-worn accelerometers, and we have pragmatically decided to report activities at relatively higher intensities by ranges of 2303–4999 and ≥ 5000 counts per minute. The purpose of using these categories is to describe physical activity at relatively higher intensities when compared to sedentary behavior. Therefore, they may not be directly comparable to moderate and vigorous intensity physical activity.

Older adults are recommended to sleep 7–8 h/night [2]. Considering sleep onset latency (the time a person lays in bed before falling asleep) increases with age [24], we deemed TIB of 7–9 h as appropriate for this age group. Participants were categorized according to how frequent they reached 7–9 h of TIB in the measurement days: UTIB (usually having TIB 7–9 h/night: ≥80% of measurement days), STIB (sometimes having TIB 7–9 h/night: 20–79% of measurement days), and RTIB (rarely having TIB 7–9 h/night: < 20% of measurement days).

Age and sex were obtained by means of a structured interview at the participant’s home. Daily nap length was calculated from the accelerometer wear-time diary. The Short Physical Performance Battery (SPPB) [25] was used to assess lower-extremity physical function during the first home visit. The battery consists of three tests that assess standing balance, walking speed over 3 m, and sit-to-stand from a chair. Each task was rated from 0 to 4 points and added up to produce a SPPB total score (ranges from 0 to 12, with higher score indicating better function).

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