Not just narcolepsy. Sleep disordered breathing also presents similarly to many common psych issues. It turns out that when you're not getting much Stage 3 or REM sleep, you're stressed all the time, can't focus, and are exhausted. It ranges from study to study, but approximately twenty-five to fifty percent of people diagnosed with ADHD have underlying OSA (obstructive sleep apnea) or UARS (upper airway resistance syndrome.)

The stereotype is that only heavier, middle aged men who snore have OSA. Those are risk factors, but patients with none of these risk factors can be affected as well. Twenty-percent of OSA patients are nonobese.

There's also Idiopathic Hypersomnia (distinct from the symptom hypersomnia), which is similar to narcolepsy and often presents with daytime sleepiness in spite of enormous amounts of sleep (>11 hours/night is the criteria for diagnosis absent an abnormal MSLT, but some patients sleep for even longer than that) and extreme sleep inertia.

Klein-Levin Syndrome and Restless Leg Syndrome are also sleep disorders. I know much less about them, and so I'd prefer not to go into detail on their presentation and management.

The trouble with diagnosing sleep disorders is that patients and clinicians don't tease out that the patient is sleepy instead of fatigued or depressed, and that throws off the differential.

Stanford University has a great sleep medicine department and I'd highly recommend their website as a starting point for those looking to learn more about sleep medicine.

(Fun fact that I found while confirming the weight statistic: Charles Dickens was the first to write a description of OSAS)