A lot depends on the related advertisers in that segment. If a lot of advertisers have timed campaigns that turn on later in the day, there's more competition later on, so (theoretically) ads that end up showing during that time period could increase in price.
The trend you're experiencing is most likely related to time zones. Google's reporting is based on U.S. Pacific Time. Depending on your niche, most of the traffic during early morning hours (i.e. 12-8a) is likely outside of North America and may pay less than U.S. advertisers. You could isolate this by using the Countries reporting in AdSense.
I noticed that my rate of payout per click steadily increases throughout the day - is this normal?
It's always been that way for me since my sites are global and predominantly in English.
I get some decent click values in the Google morning from from OZ/NZ however most clicks are from much lower-paying China > India > Middle East > Russia > then into Eastern Europe > Values increase through Western Europe > UK Pays top Dollar > Along with USA/Canada later on
I use a lot of channels and I get clicks anywhere from 3 cents to nine or ten bucks. So average EPC is all over the place. A couple days ago I had a really low day, it was like thirty cents a click for the whole day across all my sites. Yesterday it was just under two bucks a click across all sites. To me that's normal for AdSense.