The question has been asked before at How can I get a line break in a theorem headline?, but the answers seem not to work for me:
\documentclass[a4paper, twoside]{book}%[a4paper,oneside]
\usepackage[ngerman]{babel}
\usepackage[autostyle]{csquotes}
\usepackage[
backend=biber,
style=numeric,
sortlocale=de_DE,
natbib=true,
url=false,
doi=true,
eprint=false
]{biblatex}
\addbibresource{itteerde.bib}
\bibliography{itteerde}
\usepackage{amsthm}
\newtheorem{zeit}{Zeit-online}
\begin{document}
\section{2016-02-22}
\begin{zeit}[A long title as a heading for some commentary at more and longer and now or somewhere it should be broken up instead of leaving the page and it sucks?]
\begin{quote}[Waldeule]
...
\end{quote}
Die...
\end{zeit}
Text...
\end{document}
