I've been in I-T for a long time, and I've learned from the good bosses almost as much as I've learned from the bad ones.
One of my best managers, though, the kind you'd walk through fire for, left me with this gem about work-life balance and especially downtime and taking breaks:
There's never a good time. Do it anyway.
It's so simple, and points out what we know about being vital vs being irreplaceable, getting your downtime, getting the hell away from work for a while, etc.