The SPAN paradigm: combined sleep, physical activity and nutrition epidemiology and interventions across the 24 hour day

Sleep, physical activity and nutrition (SPAN) compete for the same 24 hours, influence one another, and converge on shared physiological and psychological pathways. Studying them in isolation, as most of the field still does, misses precisely the interactions that determine health. SPAN treats the day as a closed system and asks what happens when time and behaviour are reallocated within it. This project applies compositional data analysis, isotemporal substitution and related 24 hour modelling to device-measured sleep and movement combined with dietary intake and timing, testing joint associations with cardiometabolic risk, cardiovascular disease, cancer, and depressive and anxiety symptoms. Questions include which reallocations yield the largest gains, for whom, and whether the mental health and cardiometabolic optima coincide. A parallel interventional strand is available for candidates who want to design and run studies. This could test whether targeting two or three SPAN behaviours simultaneously outperforms single-behaviour interventions, whether sequencing matters, and how adherence and psychological mechanisms differ when people are asked to change a combination rather than one habit.