I've recently hired a new mid-level developer (not a junior but not a senior yet) and have been really impressed with their performance. My only complaint about them is that I get work-related emails and messages from them at 7pm and later, indicating that they are still working. We are a fully remote team working a 7-hour day, this person is regularly working 9-10 hours a day. Obviously this is great for productivity but it's not sustainable long term and I don't want them to burn out. Also, if I do need them to work out of hours on system maintenance tasks, I need them to be available. As their manager, I try to keep to only a 7-8 hour day - work/life balance is important!
In the past I've replied to late night messages with a "speak to you tomorrow" or "thanks, we'll pick it up on Monday" as a way to let them know they can leave what they're working on, but they've always said that they don't mind working late as they enjoy it and don't have much else to do.
How can I encourage this team member to work more normal hours? I'm in the UK, managing the dev team for a startup in the scale phase. We don't pay overtime as work/life balance is an important part of the company culture and on the dev team "crunching" is not part of our working practice.
EDIT - to answer some common questions/points from comments:
Yes, I know they're working these hours and not flexing their hours. There are such things as git commit logs and the green "Online" status on Teams.
I personally don't mind being available to answer questions at 9pm. I do a lot of 3rd line devops support and being as available as necessary to keep my team working efficiently and our application running is part of my job description. For a mid-level developer OTOH, their main responsibility is to produce production-quality code that meets our acceptance criteria, for 7 hours a day. I won't ask or expect them to do more than that, this is the UK and people aren't penalised for not working long hours.
Another point I forgot to raise in the question is that if someone is working extra hours, that skews our velocity and other KPIs. If they've been calculated based on the fact that one member of the team is working an extra undocumented 10 hours a week, then one day they decide they're just going to work their contracted hours from now on, that will screw up our forward planning.