There are many reasons to regret why only 24 hrs in a day
Perhaps you mean
'There are many reasons to regret that there are only 24 hours in a day'.
'There are many reasons to wonder why there are only 24 hours in a day'.
Neither is a question, so a full stop is correct.
"Thread titles should include all or part of the word/phrase being discussed." (Posting Guidelines) There are many reasons... would have been good for this thread.