Every year, I see people quitting their AoC run with frustration, because they don't measure their effort and try to complete each puzzle daily. The usual scenario is that, at some point, they block on a problem and spend longer than they should, or they can't solve the problem because they have some other thing to do. The next day, they try to solve that puzzle and another one, and as work piles up, they start doing it joylessly to catch up. After a while, it doesn't work anymore, because people are stressed, because they code poorly just to finish rather than taking pleasure in their craft. The strict calendar can become deadlines that remind of work woes.
For a person not used to sport code, AoC can take an hour to solve, sometimes more. Over the whole advent, people can easily work for 30 to 40 hours more than they are used to. In terms of work hygiene, this is not good. Not everyone has a week's worth of work to use just before the end of year.
I would advise people doing AoC to time their effort, and not hesitate to stop a puzzle and write it down for later in the year.