It can also mean a cycle that repeats itself daily. Theres 2 definitions for the word and one is a process which occurs during the day and the other says a process that occurs on a daily cycle.
Synonyms of Diurnal:
alternate, cyclic, intermittent, recurrent, regular etc etc.
So we're both right in a sense, I'm just referring to the other definition of the word!
To OP:
You should have enough stuff in this thread to start writing your paper! If you have any questions, i wrote a 12 page research paper for uni last week for this and received a 92% on it.