A reader asks about/speculates that there "might be one form of “ADD” that is simply a phenotype of ADD but that is actually caused by narcolepsy and the patient's attempt to compensate for it. This might explain why modafinil has been occasionally known to improve ADD symptoms - the reasons to engage inthe ADD behavior are abolished by the absence of sleepiness."
The new edition of Principles and Practice of Pediatric Sleep Medicine has a good chapter on "Attention Deficit, Hyperactivity, and Sleep Disorders." Conditions such as obstructive sleep apnea that present with sleepiness in adults often present with inattention and hyperactivity in children. From Principles and Practice: "...increasing evidence suggests that a variety of childhood sleep disorders are associated with inattention, hyperactivity, and cognitive impairment that could have significantly adverse effects on such important outcomes as development and school performance."
Obstructive sleep apnea and restless legs syndrome are the 2 sleep disorders that have the most evidence linking them to ADHD symptoms.
As far as narcolepsy, Principles and Practice of Pediatric Sleep Medicine mentions that "Children with narcolepsy often have problems with inattention and hyperactivity that may improve upon treatment" and "Adults who have been diagnosed with narcolepsy frequently give a history of 'attention deficit disorder' in childhood."
I have never used Modafinil (Provigil) to treat ADHD, but a quick Medline search suggests that it is effective for ADHD symptoms.
Inattention and hyperactivity can be symptoms of psychiatric disorders (including ADHD) and can be symptoms of sleep disorders. Multiple studies have linked inattention and hyperactivity to sleep disorders in children, but research is lacking in adults. However, I think it is quite possible for inattention to be a symptom of a sleep disorder in an adult.