> I attribute this to the fact that most really fun things in my life happen late - parties, dancing, playing computer games to 3am, bouts of in the zone coding, spending nights with my partner - all happening in the evening, so my body obviously tries to "optimise" its attention to when I crave it most.

I’ve personally found that 4am wake-up is driven by the same goal as 3am bedtimes: hours when everyone else is too asleep to bother you. The big difference is that it’s far easier to stay awake than it is to force yourself out of bed.