As we(?) all know, normally Codeforces Rounds start at 22:35 in China. I am in Southeast China and it's usually very late (as my parents have even put their phones away and gone sleep already)
It is very important to keep a clear mind when the time is affecting. But for me, I think I can't always keep a clear mind under this situation; sometimes I just want to f*k those problem and go to sleep as soon as possible but I can't because I have a rating.
I tried everything I can: tea, coffee, early rest. I can't tell if they work, my brain is still tired sometimes.
As a result of this, I have reached Specialist twice, Expert for six times, and CM for once(maybe there would be 5 more times). Overall, it is still on the rise, but its volatility seems too high.
I know that ratings are always floating, that's not the problem. The problem is, sometimes my performance seems to be 700 points lower than my rating or even more, and my mates have never been like this. Is it really bad to participate at night? Or just because I only worth this strange performance because I'm not strong enough.
(Strangely meaningless posts and strange English grammar. Worth a "I don't like it" from you.)
You might just participate, participate and participate. As for myself, I used to be sleepy when participating Codeforces contests for a long time, but after a few weeks, I have been usually keeping a not-bad mind. Maybe you can also find a partner who has a similar rating with you and try to keep ahead of him/her while participating. Hope these above could help you.
Please don't have sex with problems
I tend to avoid 3-hr long Div1+2 rounds. After 12:30 a.m. my brain would essentially stop working properly