Study Objectives: During adolescence, there is a precipitous decrease in slow-wave sleep (SWS) and its spectral correlate, delta power, which may reflect cortical reorganization. The temporal association between the decrease in delta power and puberty suggests that sex steroids may initiate these changes. This association has not been previously investigated. Methods: To determine whether estrogen triggers the adolescent decline in delta power, we compared delta power in 14 girls with central precocious puberty (CPP) and 6 age-matched, prepubertal controls. Five CPP participants were re-studied 7-14 months after pubertal suppression to determine if the changes in delta power are reversible after restoring a prepubertal hormonal milieu. The change in delta power was also compared between CPP participants and five historic controls from a longitudinal polysomnographic study. Results: CPP participants (6.7-10.5 years) spent 30% of the night in SWS. Delta power (3.7 × 10 6 ± 2.7 × 10 5 µV 2 ) predominated in the first 2 non-rapid eye movement episodes and decayed exponentially (tau 0.006 minutes). Age-matched controls demonstrated similar sleep staging (24% SWS) and delta dynamics (3.3 × 10 6 ± 5.1 × 10 5 µV 2 , tau 0.004 minutes). Four out of 5 CPP participants had a significant decrease (26%) in delta power after hormone suppression (p < .05), similar to historic controls. Conclusion: Using an innovative model of girls with CPP studied before and after estrogen suppression, the effects of puberty on the decline in delta power were dissociated from those of chronologic age. The current studies suggest that increased estrogen does not cause the adolescent decline in delta power and indicate that neurodevelopmental changes per se or other factors associated with puberty drive these sleep changes.