It might be because of what he or she has eaten for lunch. Many children's school lunches are packed with sugary drinks and treats that zap their energy. They get an adrenaline boost for about 40 minutes after eating, but then are tired and struggling to concentrate, nutritionists say.
So how can you pack healthy school lunches for your children to help them sustain energy during the day?
Lunch box makeover
Make sure your children's lunch has a combination of protein, carbohydrates and low-fat dairy options to help balance their energy levels. For example, instead of serving them a sandwich on white bread - an unhealthy carbohydrate high in sugar and calories - swap them for healthier alternatives like whole wheat or multi-grain bread.
A healthy lunchbox may include protein such as peanut butter; starchy foods like whole-wheat crackers, and dairy-based foods such as a cheese stick or yogurt.
Loree Taylor Jordan, author of "Fat & Furious: Overcome your Body's Resistance to Weight Loss Now," cautions parents against packing foods high in salt - such as in cookies and fruit juices.
"You can get addicted to fatty and sugary foods," Jordan says.
Some healthy alternatives recommended by other nutritionists include:
a Tortillas - such as corn or spinach - instead of white bread for sandwiches
a Vegetable potato chips, baked chips, popcorn or pretzels in lieu of regular potato chips
a Sliced fresh fruits such as oranges or kiwis, or vegetables like baby carrots
a Whole-wheat muffins or crackers
a Fruit cocktail, applesauce, mandarin or orange slices
a Trail mix that includes such combinations of nuts, dates, sunflower seeds, pretzel sticks or banana chips
a Low-fat turkey or chicken (without skin), instead of more fatty choices, such as bologna, pastrami, corned beef or ham.
After school, some children may need a snack to tide them over until dinner and keep their energy levels.
Some healthy snacks include celery topped with peanut butter and sprinkled with sunflower seeds, an English muffin with an ounce of melted cheese over it or tuna (canned in water) on crackers.
Make sure your child is getting enough to drink. Not drinking enough can lead to dehydration or fatigue.
To avoid energy slumps, bypass sugary drinks for water, low-fat or nonfat milk, unsweetened fruit juice, vegetable juices or flavored water with no added sugar.
For an additional treat, serve a fruit smoothie, which you can make yourself, using fruits like strawberries and bananas blended with orange juice, milk or yogurt.
By making sure your children eat a well-balanced diet - including healthy meals or snacks every three to four hours - they can sustain the energy they need to stay alert in school and even get an extra boost for after-school activities.