There is a fine difference between being high and overdosing and it is very important to understand the differences between the two!
When you are high, you can still have tiny pupils and look confused. But you will be able to wake up and respond if someone raises their voices or physically shakes you.
In an overdose, you will not be able to wake up, even if someone tries to.
For further details, you can also check out this video for a more detailed explanation: