Some children with ADHD do experience a greater incidence of sleepiness during the daytime. However, this is not a simple question to answer.
When adults are tired, they tend to slow down, while children tend to speed up and get more active. In a classroom environment where it is not appropriate to be overly active, falling asleep may be the result.
Although you describe your son as having a good sleeping pattern, there may be something preventing him from sleeping well ie. having a restless sleep as opposed to a fitful one.
These are mere speculations and further investigation, perhaps by a neurologist, may be warranted.
If you are not a seeking diagnosis: Perhaps if he is gaining some extra external stimuli in the absence of sound he may be able to stay awake. If your son is achieving and the hand writing is being addressed and the only area of concern is feeling tired in a quiet environment, perhaps allowing him to wear ear phones and listen to music while sitting at his desk could help; the use of a 'wobble cushion' to gain vestibular input might also assist (speak to your OT about these); something as simple as a stress ball to handle while he works may also suffice. Whatever is implemented would need the cooperation of the school, teacher and you child to behave responsibly.
The information provided does not constitute a diagnosis of your condition. You should consult a medical practitioner or other appropriate health care professional for a physical exmanication, diagnosis and formal
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