Sleep is cyclic; it comes in phases of varying depth. (http://www.helpguide.org/life/sleeping.htm) I find the difference between seven and a half hours of sleep and six hours isn't bad, but the difference between six hours of sleep and four hours of sleep is much more difficult to deal with. Looking at the diagram on the linked page, it's obvious why: sleep stages three and four are important and if you sleep less than six hours they get screwed up. More REM sleep gets messed up too, which is also important.
Day-to-day energy levels are cyclic too. Every day I want to take a nap at 3:00 in the afternoon. I'm also most energized from around 10:00 to 11:00 in the morning. It's a pattern.
A boost of energy at a certain time at night happens likely for the same reason it does during the day: levels of consciousness are cyclic.
As far as nighttime being more productive: there are fewer interruptions which means you can concentrate. It's easier when your cell phone/RSS/Twitter/blog/text/email isn't blowing up.
Day-to-day energy levels are cyclic too. Every day I want to take a nap at 3:00 in the afternoon. I'm also most energized from around 10:00 to 11:00 in the morning. It's a pattern.
A boost of energy at a certain time at night happens likely for the same reason it does during the day: levels of consciousness are cyclic.
As far as nighttime being more productive: there are fewer interruptions which means you can concentrate. It's easier when your cell phone/RSS/Twitter/blog/text/email isn't blowing up.