Sleep plays a key role in our daily lives as it allows us to memorize, regulate our emotions and be vigilant. Sleep structure is not linear but continuously transforms during development. Two independent studies are described in this work and aim (i) to capture the sleep changes in school-aged children through polysomnography recording and (ii) to evaluate the impact of a sleep cognitive-behaviour therapies (CBT) type intervention in adolescents. Study I revealed that sleep duration, N3 and REM stages duration decreased between 5 and 12 years old whereas N2 stage percentage seemed to increase in proportion to the total sleep time. An increase in the number of sleep spindles in N2 stage as well as a decrease in slow waves in N3 stage and theta rhythm in REM sleep with age have been obtained. Surprisingly, memory performance was only associated with subjective sleep duration. Emotional memories were better remembered compared to neutral ones. In Study II, cognitive functions such as vigilance and memory were improved after the sleep CBT-type intervention. In addition, sleep hygiene and affective state seemed also to improve after the intervention. Overall, it appears that the foundations of synaptic pruning and puberty are apparent in childhood. The development and mechanisms of sleep are intricate, but they provide valuable insights into brain maturation and the factors influencing overall physical and mental well-being. To promote a balanced state of health, it is crucial to increase the availability of sleep interventions and preventive measures for all children and adolescents experiencing sleep-related problems. By addressing these issues, we can potentially alleviate affective or cognitive difficulties arising from sleep problems during development.