From surviving to thriving: How sleep, physical activity, and diet shape well-being in young adults.

Healthy lifestyles are a cornerstone of optimal physical health, yet their contribution to optimal mental health is still an open question. This article investigated the relationships between three key health behaviors-sleep quality, physical activity, and dietary choices-and psychological well-being among young adults ages 17-25, a demographic known for disproportionately low levels of well-being. The research used three datasets: a cross-sectional survey (Study 1, N = 1,032) and two daily diary datasets (Study 2, N = 818; Study 3, N = 236) of young adults living in New Zealand, the United Kingdom, and the United States. Multilevel regression analyses examined how these health behaviors simultaneously and synergistically predicted well-being at the between-person and within-person levels, while controlling for covariates such as age, gender, socioeconomic status, and depressive symptoms. Additionally, Study 3 incorporated wearable device-measured physical activity to supplement self-report measures. The findings highlighted better sleep quality as the strongest predictor of well-being across all three datasets, followed by fruit and vegetable consumption, with significant associations between these health behaviors and well-being observed at the between-person and within-person levels. Physical activity also emerged as a reliable predictor of well-being, mostly at the within-person level. The use of device-measured physical activity confirmed similar patterns, reinforcing the validity of findings. Pathways linking health behaviors to well-being were mostly additive, except for a buffering pattern observed at the within-person level indicating that higher fruit and vegetable intake could potentially mitigate the negative impact of poor sleep on daily well-being. This article underscores the importance of the "big three" health behaviors in well-being among young adults, offering insights for future health interventions to improve positive psychological functioning in this population.