Background Physical activity (PA) is a recognised key determinant of future health and disability particularly in older people, the least active and most sedentary age group. The aim of this study is to examine how PA and sedentary behaviour (SB) vary over the course of the day and whether the diurnal patterns differ by age, mobility limitations, chronic conditions and body mass index (BMI). A better understanding of these variations may help refine PA interventions in the elderly.

Methods Men aged 71–91 years participating in an established UK population-based cohort study were invited to wear an Actigraph GT3x accelerometer over the hip for one week in 2010–12. Percentages of time spent in sedentary (SB, <100 counts per minute [CPM]); in light (LIPA, 100–1040 CPM) and in moderate to vigorous PA (MVPA, >1040 CPM) were derived. Multilevel models were used to estimate the associations of age, mobility limitations, chronic conditions and BMI with SB, LIPA and MVPA.

Results 1455 of 3137 men invited (46.4%) participated and provided adequate data. Men spent 73% of the day in SB, 23% in LIPA and 4.5% in MVPA (619, 197 and 39 min per day respectively). The percentage of time spent in MVPA and LIPA was highest in the morning, peaking at 10–11 am, and then declining until the evening, with the exception of a small increase at 2–3 pm. The decline in MVPA was particularly marked over the course of the day; compared to the morning the levels decreased by 43% during afternoon (95% CI –46, –41; p < 0.001) and by 83% in the evening (95% CI –85, –81; p < 0.001). Conversely, the time spent in SB increased by 9% during afternoon (95% CI 9, 10; p < 0.001) and by 21% in the evening (95% CI 21, 22; p < 0.001), when compared to morning. Men who were older, had mobility limitations, had more chronic health conditions and were obese had lower levels of PA and higher levels of SB. These factors influenced PA and SB levels in the morning more than in the afternoon and evening (p for interaction <0.05).

Conclusion Levels of activity are highest in the morning (peak at 10–11 am) and decrease during the day. SB increases through the course of the day to peak in the evening. Efforts to increase PA might focus on increasing the intensity of morning activity, attempt to enhance the afternoon activity peak, or reduce SB later in the day.