Go through these in order and stop at the first one that applies. If you are reading this for someone else, answer for them.
1
Is any of this happening now?
Call 911
Treat the situation as an emergency if:
You are becoming sleepy while driving or doing another dangerous activity
A collapse causes a serious head injury or other major injury
Weakness is one-sided, prolonged, or accompanied by speech difficulty or confusion
Someone is unresponsive, has breathing difficulty, or does not recover normally
There may have been an overdose or serious reaction to a sleep medicine
Other emergency signs, for anyone
Someone who is struggling to breathe, gasping, choking, or cannot get words out
Someone who has chest pain, tightness or pressure lasting more than a few minutes
Someone who has sudden weakness or drooping on one side of the face or body, slurred speech, or sudden confusion
Someone who is unconscious, or cannot be woken
Someone who is bleeding heavily and it will not stop with firm pressure
Someone who has had a seizure that will not stop, or does not wake up afterwards
Someone who has lips or skin that look pale, blue, grey or blotchy
Someone who you believe has taken an overdose or swallowed something poisonous
2
Is any of this happening?
Be seen today
Seek prompt medical advice if symptoms suddenly worsen, a medicine causes significant side effects, or unexplained collapses are occurring.
A first collapse should not automatically be assumed to be cataplexy because fainting, seizures, and heart problems can look similar.
Stop in a safe place as soon as possible and do not continue until it is safe and lawful to do so.
3
Does any of this apply?
Book an appointment
Arrange a routine medical appointment if you repeatedly fall asleep unintentionally, struggle to stay awake despite enough sleep, or develop episodes of emotion-triggered muscle weakness.
Assessment is also appropriate if sleepiness is affecting work, school, relationships, or your ability to travel safely.
The article also says
If sleepiness starts while driving, do not try to push through it.
Every sign above is quoted from the sofidex article on Paroxysmal sleep. If you are not sure, err upward — it is always reasonable to be checked.