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"can you shed any light? "
Few different things may be causing this. One that was most likely suspected by whoever had referred him to the sleep study is DSPS (delayed sleep phase disorder)
The sleep study can confirm or rule out this as well as other parasomnias (sleep disorders)
Another reason is that Wellbutrin being an antidepressant medication, can suppress one's REM sleep stage. The medication works on the serotonin-norepenephrine receptors. Norepenephrine as well as dopamine are involved in the fight or flight response. As a result, he may feel restless even at bed time (unable to fall asleep)
A different reason that may play a role in this is his depression. In cases of mood disorders, sleep is often disturbed. In this case, he would discuss whether or not he feels that his medication at the current dosage is effective by looking at the overall mood and symptoms that he's dealing with right now.
Problems w/ focusing/memory can also relate to depression, sleep problems as well as ADD (attention deficit disorder) The latter would have been a condition that would have existed even when he was younger rather than develop in adulthood. A person can have co-existing mental health conditions.
Feel free to clarify if this is what you were inquiring about. It is a good idea for him to have the sleep study as it can determine if his problem is related to sleep disorders or not.