A point that is not addressed in previous answers is how to set such boundaries. Particular in university teaching cultures where students are (at least implicitly) expected to be working on assignments over weekends or during evenings (which certainly obtains in the United States), it is typical for students to have queries at these times, and they can get pretty anxious if they don't hear back, because to a large extent university is not seen as a job in such cultures, but as a more all-embracing lifestyle. If you have coursework and labs all day (as a student) it may not even be realistic to ask homework questions until Friday evening, for example. And, conversely, many university professionals are indeed working at night or weekends, and it's so easy to just toss off a couple of responses so you won't have to deal with them on Monday. (I often do this myself.)
In such a setting (including the education part, not labs or the like), I strongly encourage clear communication as to when you will (and won't) reply to emails. A syllabus is a good start, but you may wish to put it in an email signature, on a learning management system, or in some other venue. And you may have to repeat it many, many times as you socialize students to this.
Yes, doing all that is annoying. Yes, it is inconvenient. But it's also providing a role model for young people in how to set their own boundaries, and that is very important for them to see. Additionally, it can help make it clear for the students (again, in my context in US undergrad education, many of them) who are used to immediate responses and are always on their own email via smartphones.
Similarly, doing so in a polite and kind fashion is critical. If you hate emails that say, "Hi did i miss anything in class i dont know why i got a c", imagine how intimidating it would be for someone who can flunk you to send an email saying, "How many times do I have to tell you not to expect an answer on Tuesday night!" Unfortunately, probably it will have to be communicated many times. But it's teaching professionalism as well as content, and if people sending emails at weird times becomes a problem (for instance if the queue becomes too long), it's important to communicate that to students.
Finally, another option would be to manage all such communication via a learning management system. That may have builtin tools to manage communication and even send auto-replies. I don't do that, because it is beyond annoying for me. But for some folks that may prove very helpful, especially if the LMS has an app that makes such responses convenient. And then you can compartmentalize a bit more. Good luck.