That is so funny what you say about how you feel and what time it is. I have been noticing that I'm more energetic in the morning and feel totally exhausted by the time 4pm hits, I just want to rest, which is tough since my shift goes until 7pm.
For me I've starting thinking of myself as a battery and I recharge when I sleep and then I get "used up" as the day goes on. I didn't experience this on other fasts since on the other ones I seemed to sleep a lot and at strange hours. This time I seem to sleep pretty regularly so I seem to be at my best in the morning and my worst at night.
However long you decide to go, each fast is a new experience and you learn more each time. I like to think that learning to fast is like building muscle: it takes time and practice.