Brief Answer:
It varies in person to person and as written below

Detailed Answer:
Hi dear
Welcome to health care magic my advice is as follows
Not getting enough sleep -- sometimes by choice -- is the most common cause of excessive sleepiness. Working at night and sleeping during the day is another. Other causes include drug, alcohol, or cigarette use, lack of physical activity, obesity, and the use of certain medications. ( if you take any please let me know)
For exact diagnosis sleep EEG is done if it shows normal sleep EEG that means normal. Neurologist can do that test
Hope this solve your query, please let me know if I can assist you further

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