The aim of the present study was to reveal how positive affect (PA), negative affect (NA), fatigue, and melatonin are inter-related in individuals with and without MDD. We used a unique dataset with up to 90 measurements of 14 depressed and 15 pair-matched non-depressed participants and the novel network analysis approach Group Iterative Multiple Model Estimation (GIMME) to reveal how affect, fatigue, and melatonin were related across time at the group- and the person-specific level. Thereafter, we investigated how individual differences in the role of melatonin were related to sleep and depression severity. PA and NA (β = − 0.47), and PA and fatigue (β = − 0.44) were related contemporaneously in the full sample. Substantial between-person differences were found. In 83% of the study participants, melatonin was related to either affect or fatigue. Those who did not have associations with melatonin in their networks had relatively greater depression severity, worse sleep quality, and lower energy expenditure. This study revealed the possibilities of network mapping for dynamic person-specific psychological and biological data. The results underline not only the presence of large heterogeneity, but also show that despite this heterogeneity, meaningful generalizations can be made regarding the interplay of melatonin with affect and fatigue in depression.
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