The availability of public logs for CQ WW CW and SSB contests spanning twelve years allows us to look at the variation in the rate at which DX QSOs are made throughout each contest over a relatively long period. (I here define DX QSOs as those between different zones; other definitions are obviously possible.)
In the plots, the number of QSOs is accumulated every ten minutes, so there are six points per hour.
I think that the results are rather interesting. I'll let you draw your own conclusions.
In regard to the 10m results, I should point out that the CW contest
occurs roughly a month after the SSB one, and is therefore considerably closer
to the winter solstice, with the concomitant decrease in the number of hours of common daylight between Europe and North America (particularly western NA). I have long wondered what would
happen if CQ WW (and CQ WPX and ARRL DX) would switch CW and SSB
weekends every year.