Children tend to sleep and wake up early and to exhibit daytime sleep episodes. To evaluate the impact of school start times on sleepiness and attention in preschool children, this study compared the temporal patterns of sleep, daytime sleepiness, and the components of attention between children aged 4-6 years that study in the morning (n = 66) and the afternoon (n = 144) shifts. The former get up 1 hr and 30 min earlier on weekdays and show lower efficiency on the sustained attention task than those who study in the afternoon. Thus, the morning shift was associated with a reduction in nighttime sleep, which might have a negative effect on children's performance in the morning, causing higher levels of daytime sleepiness and a decline in sustained attention. Because only one parameter of one component of attention was negatively affected, further studies are needed to confirm this effect on cognition.