To make sure I get out of the office at a reasonable time every day I book activities with other people, thus making sure I can't cancel, unless it's urgent of course. It just gives you another level of commitment so that you don't end up staying to "try one more thing" at work! ;) So for example, I like to book in for a tennis match in the evenings. In the wintertime, I tend to book classes in advance like yoga, that way I don't want to waste my money by not turning up. :)
Oh yea good one! I've disabled my Slack notifications between certain hours!
If I have a work related idea in non-work time I email myself and then move on with my day. I find 'exporting' it from my brain stops me going over it when I should be relaxing. So long as the email has enough information for me to pick up the idea again I don't worry about it until I'm back in the office. I do the same for home stuff while at work so I can focus. I'll set an alarm on my phone so know I won't forget to do whatever it is I've thought of at a more appropriate time.
Ooohhh I really like this idea! Must try it out! Makes sense to get it out of your head and then move on!
For those on flexible contracts - sleep when you feel like it.