This situation derives from a combination of factors, say psychologist Avi Sadeh of Tel Aviv University in Israel and his colleagues. Children tend to fall asleep at increasingly later times as they move from the second to the sixth grade, while continuing to be awakened at the same time for school. These children are thus at risk of being chronically sleep deprived. What's more, Sadeh's team finds that nearly 20 percent of kids in the second, fourth, and sixth grades have serious sleep problems that typically aren't perceived by either the children or their parents. In the study, sleep disturbances consisted of regularly being awake for at least 10 percent of the night after falling asleep or waking three or more times during the night for at least 5 minutes each time.
Scientists know little about how children develop their sleeping habits.
Earlier investigations indicated that sleep disturbances affect about one-third of babies and toddlers. Other sleep difficulties arise as the daily sleep-wake cycle gets pushed back after puberty. Grade-schoolers have attracted the least attention from sleep researchers. Sadeh's group studied 72 boys and 68 girls attending an Israeli public school. Students lived in middle-class or affluent families, most including two parents.