This may be a more agressive title than you are used to from me, but I think this is a topic that needs addressing in a very direct way. We talk a lot about burning out and needing breaks for things when it comes to US and we are constantly telling other people to take it easy... but at the same time we can often be guilty of assuming that any person that doesn't have other plans is free and available and should be giving their attention to us!
If we see a friend online when they haven't responded to a message we sent them earlier it can leave us feeling neglected. It's hard enough to assume that the person is busy when they haven't checked their phones in a while, but when we see that "last online 14:37" and we sent a message at 10am it can be hard not for us to think "well they've been online, so why haven't they responded to ME?"
The long and short of it is this, YOU are entitled to your own downtime, without having to justify or explain it to others. But the same is true the other way around. Your friends are also entitled to spend THEIR downtime however they please, and if that involves leaving you "on read" for a little while longer, it's on US to respect that and that my friends is why the more aggressive title, because I believe the responsibilty for changing this situation lies with us!