The sleep disorder
Apparently, feeling sleepy all day can take a more troubling form as a long-term neurological disorder called Narcolepsy.
Quoting from Wikipedia,
There are two main characteristics of narcolepsy: excessive daytime sleepiness (EDS) and abnormal REM sleep.
Dr. William C. Dement was one of the first people who approached the topic of “sleep” more seriously. Incumbent medical institutions across America were mostly indifferent. The fruits of the labor he started in the 1950s were borne in the 1990s after he provided extensive experimental evidence. Insomnia was pervasive throughout the population and it was alarming that nobody paid attention. To his due recognition, he ran a highly popular course on this topic.
His work is now curated at The Sleep Well website with links to other resources.