Difficulty staying awake can have various causes, ranging in severity from 'worrying' to 'needs attention'. Finding the true cause means ruling out or confirming each possibility – in other words, diagnosis.
Diagnosis is usually a complex process due to the sheer number of possible causes and related symptoms. In order to diagnose difficulty staying awake, we could:
Do you have difficulty staying awake after midnight?
Possible responses:→ Yes, always
→ Usually / I only stay up for special occasions
→ Sometimes / I'm about average / don't know
→ No, I regularly stay up into the small hours
→ Not at all, I'm a real "night person"