This study assessed the effects of modest sleep restriction and extension on children's neurobehavioral functioning (NBF). The sleep of 77 children (age: M=10.6 years; range=9.1–12.2 years) was monitored for 5 nights with activity monitors. These children (39 boys and 38 girls) were all attending regular 4th- and 6th-grade classes. Their NBF was assessed using computerized tests on the 2nd day of their normal sleep schedule. On the 3rd evening, the children were asked to extend or restrict their sleep by an hour on the following 3 nights. Their NBF was reassessed on the 6th day following the experimental sleep manipulation. Sleep restriction led to improved sleep quality and to reduced reported alertness. The sleep manipulation led to significant differential effects on NBF measures. These effects may have significant developmental and clinical implications.