Currently the National Health Service Blood and Transplant (NHSBT) in England allows men to donate blood up to every 12 weeks and women to donate up to every 16 weeks. In recognition of a likely future increase in demand for blood transfusions the Department of Public Health and Primary Care (DPHPC) at Cambridge University is conducting a study into the safety and acceptability of decreasing intervals between blood donations, with a view to informing future blood collection policy. The INTERVAL study is a randomised, multi-site trial involving 50,000 male and female donors from donor centres of NHSBT throughout England, in which men and women are assigned to groups of differing blood donation intervals (12, 10 and 8 weeks for men; 16, 14 and 12 weeks for women). Key outcomes include donor quality of life, ‘deferrals’ due to low haemoglobin (and other factors), iron status, cognitive function, physical activity and donor attitudes. INTERVAL is using the AX3 band to assess the impact of blood donation interval on the physical activity levels of 6000 participants (1000 of each arm of the study).