You can compare the data from this study with the data generated at Johoe's Mempool Statistics page: https://jochen-hoenicke.de/queue/#1,2w. Here's a random 2 week sample I took from last September (click for full size):
The dates along the bottom are at midnight (00:00) UTC. So you can see from this sample, there is a spike in fees on most days between around 10:00 and 18:00 (when Europe and then later America are awake), at which point they start reducing, and hit a low point around 01:00 - 04:00 UTC. This certainly ties in with the data from OP's study.
It's something I've casually noticed, but never given much though to. But considering it now, I wouldn't have expected such a constant and similar drop given that China has more people than Europe and North America combined.
The dates along the bottom are at midnight (00:00) UTC. So you can see from this sample, there is a spike in fees on most days between around 10:00 and 18:00 (when Europe and then later America are awake), at which point they start reducing, and hit a low point around 01:00 - 04:00 UTC. This certainly ties in with the data from OP's study.
It's something I've casually noticed, but never given much though to. But considering it now, I wouldn't have expected such a constant and similar drop given that China has more people than Europe and North America combined.