Riders were started potentially every 5 minutes. One rider, Harish Narayanaswamy, started at 6:15, 15 minutes after the official cut-off (but he still finished with daylight to spare); all others started within the official 4 am to 6 am start window. So I binned the riders by start time then did a weighted linear regression of the average net times. The result is plotted here:
Not surprisingly, faster riders tended to start later. Or perhaps riders who started later tended to be faster due to less riding in pre-dawn darkness, although I suspect more of the former. Obviously it takes a special motivation to get out there for a 4 am start, foregoing the advantage of the 5 am mass-start, and a nice source of such motivation is fear of riding after sunset on much more heavily trafficked roads, or fear of missing the 10:30 pm cut-off.
Note the slope is < ‒1 (value = ‒1.69, standard error = 0.21), which means riders who started later in the morning tended to finish earlier in the evening, so it isn't a simple matter of riders pushing up against sunset or the cut-off.
Interestingly only 59% of the 194 finishers started with the main 5 am group. 36% started earlier and 6% started later.