Brain maturation and sleep patterns evolve throughout childhood, intricately influencing cognitive functions. However, it remains unclear whether changes in sleep and cognition follow similar or distinct trajectories as a function of age and gender during childhood. We included 61 healthy children (30 boys and 31 girls), aged 5-12 years old, who completed a visual memory task and a sustained attention to response task (SART), before and after undergoing one night of polysomnography at home. Our findings revealed large age-related associations in girls, with N3 and REM durations decreasing and sleep spindle frequency during N2 increasing across development. Conversely, these patterns were not observed in boys. Moreover, a significant interaction showed a shift in delta power topography from posterior to anterior regions in girls compared to boys. Alongside these sleep changes, girls exhibited a predominant excitatory pattern of brain activity during wakefulness as evidenced by a reduction in resting EEG slope. Regarding cognition, we found a large correlation between the increase in sleep spindle frequency in girls and morning accuracy on the SART. Overnight memory consolidation did not vary with age or gender. Taken together, these findings suggest an earlier onset of brain maturation in girls, reflected by less deep sleep, faster sleep spindles, frontal shift in delta power, and greater cortical excitability during wakefulness. This pattern in girls challenges the notion that developmental modifications of sleep are minimal during childhood. How closely may these changes relate to puberty or timing remain to be established in future longitudinal studies.