With my very random sleep rhythm I get to play at almost every time. But sometimes I seem to only lose at my rank, and sometimes I am playing quite unexperienced players, giving me easy wins.
The main reason for the skill difference can be found when looking at the clock. The hardest games seemed to occur at about 10:00-14:00. Being at CET, you add 7 hours to get to KST, and you have it. Evening in Korea. Obviously this is no real suprise, but good to keep in your mind, that when you always play at that time, you might be a higher rank during other parts of the day.
Another thing that I have experienced, is that on the weekend it is easier than during weekdays. Weekend is free time for most people, so many people come online that can only play during weekends (or only very limited on the weekdays). These players with jobs have less time to play than the pupils/students/unemployeds hardcore gamers. A more casual player base leads to lower average skill level.
Furthermore there seems to be a difference between maps. I mostly lost on Blue Storm. But this might come from me not being able to play the maps. Hosting non-motw maps that I want to play at that point I usualy got worse players. Easily explainable because higher level players want to get out of the lower ranks quickly.
I mostly just host public games and wait for people to join, but when I joined one those few times those players seemed to be worse than the players joining my games. But out off 3 or 4 times you can't make an observation.
Last thing I noticed, but only slightly, so it might also just be random is that evening here is a bit harder than the night, evening in America.
I won't go into the beginning of the season or other factors like TSL, which also have an impact on this, but are not subject to my 'research'.
Problems with my observations:
-Small sample size during long time period (a bit over 150 games in 3 seasons)
-Subjective estimation of play strength based on how I did against them
Problem #1 can easily be covered by playing more getting different observations.
Problem #2 is however really tricky. Since there is no way to measure a players skill; we don't have a power level that might be over a certain value. There simply is too much to Starcraft game to measure the skill of a player. Even when just comparing two players it is hard to tell whos better, unless there is a quite large skill gap.
If anyone observed similar things go ahead and tell me .