But I soon realized that headaches are more related to the sleep mode and the number of hours spent in front of the monitor screen.

Yes, I think so too. People like to make external causes responsible first, rather than looking for them in their own person. :-)
Sleeping is very important for the immune system. I have to admit that sometimes I don't sleep sufficiently.
On top of that, there may also be one's own vulnerability.

without trying to explain other things.

Very strange. The regularity is very noticeable. I can't believe this hasn't been followed up. But may be, this wasn't die question in the study...

for economic reasons I have nothing against the winter that has been until now :)

This is exactly what I think! ... at least this year ... ;-)