0236 Development of the Hippocampus in Preschool-Age Children in Relation to Daytime and Nighttime Sleep

Abstract Introduction Development of the hippocampus is critically supported by sleep. Although previous research highlights the importance of both daytime and nighttime sleep for cognitive and neural outcomes, less is known about how hippocampal development across early childhood shapes sleep behavior, particularly in the shift away from daytime naps that is characteristic of the preschool years. As the hippocampus matures, children may consolidate memories more readily without relying on daytime naps, meaning that hippocampal development may drive the transition from biphasic to monophasic sleep (Spencer & Riggins, 2024). Methods Young children (N=52; M=3.97 years, SD=0.68 years, Range=3.05-5.98 years, 25 males) wore actigraphs for 3-16 days to assess sleep patterns. Nap frequency was variable, ranging from 7.4%-100% of days with naps (M=69.1%; SD=26.9%). 24-hr sleep time was calculated as the sum of both nap and overnight sleep and a nap ratio reflects the proportion of 24-hr sleep obtained in naps. T2-weighted MRI scans were processed using ASHS (Automatic Segmentation of Hippocampal Subfields) software to segment the CA1, subiculum, and CA2-4 with dentate gyrus (CA2-4+DG); volumes were adjusted for participant age. Results Correlations revealed that nap ratio was negatively correlated with CA2-4+DG volume, r(50)=-.28, p=.04. This was not the case for the CA1, r(50)=-.15, p=.30; or subiculum, r(50)=-0.13, p=.35. In an ANOVA predicting raw CA2-4+DG from 24-hour sleep, nap ratio, age, and sex; sex significantly predicted CA2-4+DG volume, F(1,46)=4.71, p=.04. Nap ratio was a marginal predictor, F(1,46)=3.82, p=.06, but neither 24-hour sleep time, F(1,46)=2.09, p=.15, nor age, F(1,46)=0.59, p=.45, was predictive of CA2-4+DG. Conclusion Hippocampal subfield volumes may be linked to nap ratio of overall sleep. These connections align with a potential link between developing hippocampal subfields and a reduction in the need for daytime memory consolidation via naps. Given that the CA2-4+DG and subiculum develop later and more variably than the CA1 (Canada et al., 2021), those subregions may be the catalyst for the also variable nap transition timing. Support (if any) Funding by: NIH R01 HL164628