I've noticed that too: poor decision making, can't solve obvious tactic problems.
Here's a trick, try to solve some tactics puzzles after a 20 min light jog, you'll be amazed by the results :)
Being tired.
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I get runs up and down regardless. I'm tired now and just recovered 300 points I lost yesterday, when I just wasn't seeing anything. You definitely have to be focussed and not in a 'can't be bothered' mood.
I've noticed this too. If it is late then I now avoid tactics trainer or playing games as I am at least 100 points weaker at night then I am in the day, maybe more. This is a pain because I want to keep studying and studying but it is a case of diminishing returns.
I sometimes play blitz or bullet at night because I'm not that interesting in the rating for that, or play unrated games or read chess books, but overall when I am feeling even a little tired nothing much productive is going on. I wonder what the best thing to do/study in terms of chess improvement is under these conditions?
I know it’s obvious really but I’ve been surprised at what a difference being tired makes to particularly tactics trainer and missing the obvious. I’ve actually stopped playing TT when I’m tired as I don’t like feeling stupid. There are probably some lessons to be learnt regarding real life too in this respect.