A simple apology.
Here is a simple way of saying what happened.
I'm sorry. I overslept.
This explanation might avoid punishment once.
One interpretation: An apology is a promise.
Many people believe that a genuine apology includes:
- acknowledging that you were wrong.
- admitting to what you did wrong.
- genuinely trying to understand how you harmed (or inconvenienced) others.
- thinking about what you did, to find what you could have done differently.
- consistently endeavoring in the future to avoid making that mistake, so that if a similar situation occurred in the future, you would not harm (or inconvenience) others.
"I'm sorry", with an acknowledgement of what you did wrong, can be interpreted as such a genuine apology. If you make the same mistake again (and inconvenience the same person(s) again in a similar way), people will doubt whether your apology was sincere.
Thus, "I'm sorry" raises the stakes -- it can change an issue from a matter of politeness or inconvenience to a matter of honor, honesty, and integrity.
Another interpretation: An apology is a polite noise.
Many other people think "I'm sorry" is just a polite noise people make to acknowledge inconveniencing someone. Unfortunately, you cannot know in advance whether the person you are talking to interprets apologies as "mere words", or as sincere promises to avoid making the same mistakes.