Seasonal light changes amygdala size, but not mood in the same study
In a large sample of over 10,000 UK adults aged 45–79, longer photoperiod was associated with larger total amygdala volume and larger volumes of every amygdala subregion in both hemispheres [4]. The subregions most strongly linked to photoperiod included the lateral nucleus, basal nucleus, central nucleus, cortical nucleus, and medial nucleus, among others [4]. This suggests the whole amygdala, not just one part, responds to seasonal light cues in structure.
However, in that same study, photoperiod was not significantly associated with mood scores, and mood scores were not associated with amygdala volumes [4]. So while seasonal light may reshape the amygdala, this particular study did not find that those size differences translate into mood differences in healthy adults. The authors note this is the first demonstration of a photoperiod–amygdala volume association in humans, so the finding is new and needs replication [4].
Activity in amygdala subparts shifts with season and affective state
A 2025 study examined responses of 252 amygdala subparts in relation to season and affective state [2]. The abstract indicates that longer-term photoperiod effects may reach the amygdala indirectly through the suprachiasmatic nucleus (SCN), the brain's master clock [2]. This is a proposed pathway, not a confirmed mechanism, and the abstract does not specify which subnuclei are most tied to mood tracking [2].
In animals, the medial amygdala shows photoperiod-dependent changes: in Siberian hamsters, day length alters androgen sensitivity in medial amygdala subnuclei [3]. This is consistent with the idea that the medial amygdala is sensitive to seasonal light cues, but it comes from a seasonal breeder and cannot be directly translated to human mood. It does suggest, however, that subnuclei within the medial amygdala may be worth watching in seasonal studies.
Which subnuclei track mood? Clues come mainly from bipolar disorder
The clearest subnucleus-specific mood links come from a study of 127 bipolar disorder patients and 131 healthy controls [1]. In that study, depression severity correlated with functional connectivity between the right lateral amygdala and parts of the right and left hippocampus [1]. Depressed bipolar patients showed decreased connectivity between the right lateral amygdala and right anterior and posterior hippocampus compared with non-depressed patients [1]. Manic patients showed increased connectivity between the left medial amygdala and the shell of the left nucleus accumbens [1].
These findings indicate that the lateral amygdala may be more tied to depressive states and the medial amygdala to manic states, at least in bipolar disorder [1]. But the study did not find differences between the whole bipolar group and healthy controls, and it did not examine seasonal or photoperiod effects [1]. For healthy adults specifically, no subnucleus has been confirmed to track mood in the same way. The bipolar findings are a useful lead, but they are not evidence about healthy mood variation.
Sources used in this answer
Functional connectivity of the amygdala subnuclei in various mood states of bipolar disorder
In bipolar disorder, depression severity correlates with right lateral amygdala–hippocampus connectivity, and mania is linked to left medial amygdala–nucleus accumbens shell connectivity, but no whole-group difference from healthy controls was found.
Mood, season and light within the amygdala
Amygdala subpart responses vary with season and affective state, with a proposed indirect photoperiod pathway through the suprachiasmatic nucleus, though specific mood-tracking subnuclei are not clearly identified.
Photoperiod-Dependent Response to Androgen in the Medial Amygdala of the Siberian Hamster, Phodopus sungorus
In Siberian hamsters, photoperiod changes androgen sensitivity in medial amygdala subnuclei, showing seasonal plasticity in an animal model.
Amygdala and subregion volumes are associated with photoperiod and seasonal depressive symptoms: A cross‐sectional study in the UK Biobank cohort
In over 10,000 UK adults, longer photoperiod was associated with larger total amygdala and subregion volumes, but photoperiod was not associated with mood and mood was not associated with amygdala volumes.