Respecting the end of the week.
It's tempting to carry our work into the weekend especially when we don't have a clear cutoff day and time for the end of the week.
But if we work over the weekend, there is no beginning of the week either. Instead of pushing yourself for another couple of days, you are looking at a 12 day week.
Shutting down for the week allows us to start afresh on Monday and to become more effective by turning the week into discrete units whose output can be measured.