Loneliness is a complex social state with negative societal and health effects.

We know much about group-level differences in loneliness.

Between-person estimates do not capture all within-person dynamics.

The person is the basic unit of socio-psychological organization.

Within-person designs may create a more theoretically consistent research landscape.

Loneliness arises when individuals perceive their social relationships as inadequate. Extensive research has linked loneliness to physical and mental health risks, as well as increased mortality. However, despite these well-documented consequences, several key aspects of loneliness remain surprisingly underexplored — largely due to the predominant reliance on between-person research designs. While these designs are invaluable for identifying interindividual differences in loneliness, they do not provide insight into its subjective nature or the specific circumstances under which loneliness arises and persists over time. To fully grasp the complexity of loneliness, we argue that research must move beyond traditional between-person approaches and more routinely integrate within-person perspectives. This shift will provide deeper insight into how loneliness unfolds within individuals, ultimately advancing our understanding of its dynamic nature.

© 2025 The Author(s). Published by Elsevier Ltd.