Not getting enough sleep -- sometimes by choice -- is the most common cause of excessive sleepiness. Working at night and sleeping during the day is another. Other causes include drug, alcohol, or cigarette use, lack of physical activity, obesity, and the use of certain medications.
But nodding off when you want or need to be awake may also be caused by an underlying condition. Depression or a sleep disorder such sleep apnea are common causes of problem sleepiness.
Sleep apnea is becoming a more common cause of sleepiness in children and adults. Because your breathing is interrupted, so is your sleep, leading to sleepiness during school, work, or other activities... Falling asleep in traffic or at work is obviously less than ideal. Students that are trying to stay awake for classes and exams is not good for the grades or the child.