I’m a night owl, I like to do thought work after the day of busy meetings. I don’t want my direct reports feeling like I expect them to reply at 11pm when I send them an email, or on Sunday if I’m working. So I schedule it to arrive in the morning or in the next work day instead.
You can tell them until you’re blue in the face, but actions speak louder than words.
By scheduling the message to send in the morning you are just setting the expectation to be awake and respond then, you aren't removing the issue you mentioned, you are just picking a different time to set the expectation.
My experience has been that simply telling them not to feel obliged to respond doesn’t really cut it, and people feel obligated regardless. This may be a cultural thing too, and again will probably differ company to company and region to region.
If one of my team sends too many emails at weird hours I’d make a point of talking to them about their workload and life balance at an upcoming 1:1 to make sure things aren’t getting too off track.