There is a list of things I go through whenever the kids start wilting and lagging. 

The phone calls from the school increase saying to come and collect them they are sick. 

They become lethargic,  nauseous, have trouble sleeping  and concentrating on their work. 

Frequent bugs coming through are one thing, kids after all are the apex plague monkeys ensuring exposure to every flu cough and cold making the rounds. However at times it can extend beyond flu season and not be shared by the entire household. Then you begin with blood tests checking for;

• Iron
• Vitamin D 
• Thyroid function
• Blood sugar and others

AAddress those issues then speak to specialists. 

One such specialist to always include in rounds is your friendly local optometrist most of us should have our eyes checked every year for school aged kids and maybe every 2-3 for adults unless there is diabetes or other health condition which requires visits more often. 

What may be revealed can of course vary.  3 of my children now have glasses one also now needs to do exercises twice a day for her eyes. 

These are called Convergence problems. 

After only a week of exercises the frequent phone calls from the school to collect her have stopped. 

And can be helped even with a $5 pair of. Magnifying glasses in some cases from your local dollar shop. 

Just another way to reduce the number of small challenges our children face that they can focus in class.