No comma.

If a comma were used, I would not understand its purpose.

I would use one because "in the morning" is functioning as an introductory prepositional phrase. That's what my senior-year English teacher from hell drilled into us. However, I'm in the minority nowadays, and the Purdue writing lab (below) gives some practical advice. Unfortunately, most people omit the comma after even very long introductory phrases.


From https://owl.english.purdue.edu/owl/resource/6...
When to use a comma

Introductory elements often require a comma, but not always. Use a comma in the following cases:

After an introductory clause.
After a long introductory prepositional phrase or more than one introductory prepositional phrase.
After introductory verbal phrases, some appositive phrases, or absolute phrases.
If there is a distinct pause.
To avoid confusion.

When not to use a comma

Some introductory elements don't require a comma, and sometimes the subject of a sentence looks like an introductory element but isn't. Do not use a comma in the following cases:

After a brief prepositional phrase. (Is it a single phrase of fewer than five words?)
After a restrictive (essential) appositive phrase. (See our document on appositives.)
To separate the subject from the predicate. (See below.)

Comma optional.

hat es eine Auswirkung auf die Satzbedeutung?

No, but maybe in the case of longer phrases on its understandability, see hbberlin's answer.

Also agree; one can/needs to be pragmatic when setting commas in English I'd say.

Thanks for your answers! I think I’ll handle it like hbberlin recommends with his citation. As “In the morning” is a short introductory prepositional phrase of fewer than five words, I won’t use a comma. Keeps it easy ;-)

Unfortunately, that "fewer-than-five-words" rule doesn't always apply. Having said that, I'd like to point out that hbberlin even indicates as much.

I'd use a comma, too :-)