Great thread topic. I think many of us (myself included) deal with this same phenomenon and I think there are several factors playing into it. My current signature on this site is to remind myself (and perhaps others) to stay aware of how daily habits and actions affect mind state.
Possible contributing causes
1) If you have any sort of career that keeps you busy, chances are good you eat like sh1t on a regular basis. Not always unhealthy food, but infrequent meals, meals that are way too large, far too many carbs, too few fruits & vegetables. To boot you probably also have a regular intake of caffeine, and sometimes sleep-enhancing medications in the evening. The cumulative effect of all this is roller-coaster vitality levels.
2) As Joekerr suggests, night-time is peaceful. It's much easier to be creative and feel energetic with the responsibilities and deadlines of the daytime off your shoulders.
3) Unnatural light exposure. Ever go camping, or spend most of a weekend outdoors? I bet you felt tired and ready to crash fairly early at the end of the day. It was probably some of the best sleep in a long time. A lack of sun exposure coupled with constant artificial light after dark really messes with our biology. Not to mention the fact that most people block sunlight from their bedroom.
4) People feel more anonymous at night. It's not really rational, but as you said Analytic, the darkness masks your faults and imperfections, as you can shed your work persona for the comfortable freedom of your personal sphere. No judgments, no accountability, and it gives the imagination some breathing room.
5) You're experiencing a lack of extended physical activity to naturally fatigue the body.
6) Keeping "college" hours. That weekend party until 3 am and the alcohol-induced sleep until noon the next day mess up people's rhythm far more than they think.
7) Without early exercise during the day, it takes the mind and the metabolism several hours to get up to full throttle.
8) Distractions that are "off limits" during the day are available at night, increasing your alertness instead of helping you wind down. It's like when you were a kid and falling asleep for the last hour of school, only to feel like a million bucks when you got out and could get back to playing video games.
9) People tend to compartmentalize the value of their time, as if the late night hours are any less valuable than daytime hours. That extra hour of late-evening creativity has to be accounted for somehow, whether it's in lost sleep or a late start to tomorrow. The compound effect of this can really do some serious damage to your daytime effectiveness, especially toward the end of the week.
I really think that the malady described in this thread is a seriously overlooked problem for people in the Information Age. We have more choices, distractions, stimuli, expectations, freedoms, convenience, protections, concerns, and fears than any generation of humanity ever has. It would be interesting to see some sort of comprehensive research, if any study has ever been conducted on this topic.