Introduction: Circadian misalignment (e.g., shift work) is associated with worsened mood and mood has been shown to have a circadian rhythm. Notably, rhythmicity has been shown for feelings associated with reward-related motivation (positive affect) but not threat-related motivation (negative affect). We tested whether or not there exists a circadian rhythm in mood as assessed by the complete Positive and Negative Affect Scales (PANAS) and a questionnaire with validity in both psychiatric and normative populations. Methods: 15 participants without chronic medical or psychiatric illnesses were studied (aged 41-63 years; 9 females). Following 1-3 weeks of a regular sleep/wake schedule participants underwent a laboratory forced desynchrony protocol that distributed all scheduled sleep/wake behaviors evenly across the circadian cycle (achieved by scheduling 10 identical, recurrent 5 h 20 min 'days' in dim light thereby desynchronizing the circadian and behavioral cycles). POMS-B and PANAS were completed ~30 min. after waking. Scores were converted to z-scores and analyzed by cosinor analysis. Circadian phase at each mood assessment was determined relative to the salivary dim light melatonin onset (DLMO, 3 pg/ml threshold). Results: Both POMS-B mood score and PANAS positive affect items exhibited robust circadian rhythms (both p <0.001) with the worst mood and lowest positive affect occurring during the biological night (~6 hours after the DLMO or ~2:45 am) and the best mood and greatest positive affect at the end of the biological day (~2 hours before the DLMO). Negative affect mirrored positive affect (p = 0.01) with a peak ~ 6 hours after the DLMO and minimum ~6 hours before the DLMO but with an amplitude that was 39% of the rhythm in positive affect. Conclusion: Mood has an endogenous circadian rhythm driven principally by positive affect but also influenced by negative affect. Normative and clinical assessments of mood should take these rhythms into account. These rhythms may underlie the association between circadian misalignment and mood disorders and differences in rhythmicity of positive and negative affect might contribute to mood disorder risk. Support (If Any): R01 HL125893 (to SAS), NCC 9-58 and F32HL131308 (to SST) and UL1TR000128 (to Oregon Clinical & Translational Research Institute).
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