Looks like you already addressed that merge time != commit time. That being said, while it's hard to make a blanket statement about what part of the working process a commit falls, I'd venture a guess that it's usually at the end of a period of work. Given that, the commit time covers an indeterminate period of time before the commit's time. So having no commits at 6 AM doesn't mean the person isn't working at 6 AM, but it probably means they aren't working at 5 AM or 4 AM.

But that all being said, we only have a handful of people worth of data points and who's to say that Linus isn't someone who writes his code the previous day and commits it the next. Or that Guido isn't someone who makes lots of small commits as he's working. Hell, maybe they all squash multiple days worth of commits leaving it with a timestamp for a totally different time.

tl;dr: Yeah, I don't know that we can glean that much super reliably from this data set.