For focus to be optimal, your child needs to have support. In the book, we talk about how important it is to have good sleep (both enough hours as well as good quality sleep), good nutrition, and exercise. You can think of these as the three legs of a stool.
When people are sleep deprived, either from too little hours (very common in teens) or from poor quality sleep like waking in the middle of the night for different reasons, one of the first things that goes is their focus. Even if they don't "feel tired," if you were to do formal tests of focus, that is one of the first things to suffer.
The second leg is nutrition. Every study that has ever been done on the subject shows that if you eat breakfast you have a better chance of doing well in school. Yet so many kids will skip. Sometimes they are too tired, or too rushed, but there should always be time to grab something. Pound for pound your brain is the most metabolically active part of your body. It needs some good fuel to work.
Finally, exercise is wonderful for focus, and also helps knock out things that can interfere with focus such as anxiety or depression. It's a self esteem booster, and, to return to the beginning of this, helps with sleep.
Sleep, nutrition and exercise really hold a person up! Without any of these, you can still get through your day of course, but your focus might be on the wobbly side!