I'm in California, usually, but have often observed HN through the California night -- either because of my own odd online hours, or trips to distant time-zones.
It's true there's never total quiescence, but the pace of actions changes by a noticeable factor. (Without going to the data, I'd guess 5X from trough to peak over a day's cycle, and a somewhat smaller weekend-to-weekday difference. Holidays and nice bay area weather also play a factor.)
I've found optimal submission time to generally be midday london time, you catch the european lunch-time traffic and the US wake-up/get-into-work traffic. I don't think traffic from anywhere else is heavy enough to matter.
I'd be interested to know what the hourly fluctuation for HN is actually like, on account of having readers all over the world.
I'm in Australia, so your example "submitted at midnight" California time[1] means submitted at 6pm my time. Also 8am London time, 11am Moscow time. :).
[1] I'm going to go ahead and assume you're in California. ;)