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Full list of author information is available at the end of the articleSedentary behaviour – i.e., low energy-expenditure activity undertaken in a sitting or reclining position [1] – is asso-ciated with adverse physical and mental health outcomes [2-4]. Even among the physically active, time spent sedentary is associated with higher waist circumfe-rence, blood pressure, and 2-hour plasma glucose [5,6]. health effects even where physical activity recommenda-tions are met [7,8], and so sitting time is now recognised as a health risk factor independent of physical activity [9]. Physical activity guidelines are increasingly incorporating recommendations to limit sedentary time [10]. Older adults are more likely than other age groups to be sedentary [11,12]. TV viewing, one of the most preva-lent leisure-time sedentary activities [13], is particularly common in older adults. It is estimated that adults typically * Correspondence: b.gardner@ucl.ac.uk1Health Behaviour Research Centre, Department of Epidemiology and PublicAbstrac