Morning resolutions, evening disillusions: Theories of willpower affect how health behaviours change across the day

People may be more or less vulnerable to changes in self-control across the day, depending on whether they believe willpower is more or less limited. Limited willpower beliefs might be associated with steeper decreases in self-control across the day, which may result in less goal-consistent behaviour by the evening. Community members with health goals (Sample 1; N = 160; 1814 observations) and students (Sample 2; N = 162; 10,581 observations) completed five surveys per day for one to three weeks, reporting on their recent physical activity, snacking, subjective state, and health intentions. In both samples, more limited willpower beliefs were associated with less low- and moderate-intensity physical activity, particularly later in the day. Limited willpower beliefs were also associated with more snacking in the evenings (Sample 1) or overall (Sample 2). These behavioural patterns were mediated by differential changes in self-efficacy and intentions across the course of the day (in Sample 1), and the above patterns of low- and moderate-physical intensity held after controlling for related individual differences, including trait self-control and chronotype (in Sample 2). Overall, more limited willpower theories were associated with decreasing goal-consistent behaviour as the day progressed, alongside decreasing self-efficacy and weakening health-goal intentions.