Day-to-day fluctuations in motivation drive effort-based choices

Internal states like motivation fluctuate substantially over time. However, studies of the neurocomputational mechanims of motivated behaviour have failed to capture this. Here, we examined how naturalistic ups and downs in state motivation influence the subjective value of reward and effort. In a novel microlongitudinal design (N=155, state timepoints=3344, decision-making tasks=845), we captured fluctuations in state and effort-based decision-making using smartphone-based, momentary assessments as people went about their daily lives. We found that both state and trait have independent and multiplicative effects on decision-making. State-behaviour coupling was particularly pronounced in individuals with higher trait apathy, meaning that their choices were even more state dependent. Using computational modelling, we demonstrate that state motivation prospectively boosted reward sensitivity, making people more willing to exert effort in future. Our results show that day-to-day fluctuations in state and cognition are tightly linked and critical for understanding fundamental human behaviours and mental ill-health.