Getting kids to bed on time can be a constant battle for some parents, but it might just make a difference between an A and a B on your child’s next report card. The Journal of Epidemiology and Community Health reports that irregular bedtimes can suck the energy and smarts out of kids’ brains, rendering them not ready for learning the next day.

The study of more than 11,000 kids in the United Kingdom recorded bedtimes at ages 3, 5 and 7. One in five 3-year-olds went to bed at odd hours, and also seemed to be the most sensitive to varying bedtimes. Both boys and girls showed lower scores in reading, math and spatial awareness, suggesting that the toddler age could be an especially tricky time for mental development.

But it’s incredibly important for kids to get sufficient sleep so they develop regularly. Poor sleep can cause not only poor grades, but also weight gain, accidents, sports injuries, poor judgment and surly moods. Anyone who’s ever had to wake up a sleeping teenager can attest to that.

Toddlers need 12 to 14 hours; preschoolers, 11 to 13; school-age kids, ten to eleven; and teenagers, eight to nine. So to make sure kids are getting their share of shut-eye, stick to a firm bedtime, and explain that sleep will help them grow strong and smart. Limit caffeine, encourage exercise and keep electronic gadgets out of the bedroom — the light can disrupt critical REM sleep.

But when kids “just can’t fall asleep,” try this trick to help them doze off. The “relaxing muscles” activity is effective for kids ages five to seven. Tell junior to tighten and relax each muscle one at a time, from face, hands and fingers, to tummy, ankles and toes. Talk him or her through it the first couple times, and then encourage them to try it on their own. They’ll be dozing in dreamland in no time.