Diminishing returns to work

How Many Hours Should you be Working? (via Pawel Hytry)

– There’s a point of diminishing returns to work, and it’s closer to 40 hours a week than we’d think
– Meeting with employees is productive; meeting with consultants or outsiders is not

But how actionable is this?  While I completely understand diminishing marginal productivity, it’s really difficult to observe in the moment.  How can we build in triggers to know when our productivity is going down?