The current study examines the relationship between prolonged loneliness, physical health, and sleep among young adolescents (10 - 13 years; N = 1214; 53% girls). Loneliness was measured at 10, 12 and 13 years of age along with parent-reported health and sleep outcomes. Using growth mixture modelling, 6 distinct trajectories were identified: ‘low increasing to high loneliness’ (n = 23, 2%), ‘high reducing loneliness’ (n = 28, 3%), ‘medium stable loneliness’ (n = 60, 5%), ‘medium reducing loneliness’ (n = 185, 15%), ‘low increasing to medium loneliness’ (n = 165, 14%), and ‘low stable loneliness’ (n = 743, 61%). Further analyses found non-significant differences between the loneliness trajectories and parent-report health and sleep outcomes including visits in to health professionals, perceived general health, and sleep quality. Those findings contrast with the adult literature, and the growing body of youth research, and highlights the importance of informant choice when reporting health. The implications of the findings for future empirical work are discussed.