What does it mean that attention is a finite resource?

You wake each day with a limited reservoir of focused attention — roughly three to four hours of genuine deep work — that depletes with every act of sustained concentration and cannot be refilled by willpower alone.

Example: An engineering lead spends the morning in three back-to-back design reviews, each requiring careful evaluation of tradeoffs. By 1 PM she sits down to write the architectural RFC she's been planning all week — and stares at a blank document for forty minutes, producing nothing. Her attention budget was already spent. The problem wasn't motivation or discipline. She scheduled her most demanding creative work after she'd already burned through her daily supply of directed attention.

Try this: For one workday, keep an attention log. Set a timer to ping every 90 minutes. At each ping, rate your current focus from 1 (scattered, unable to sustain a single thread) to 5 (locked in, unaware of time passing). Note what you did in the prior 90-minute block. At end of day, plot the four or five ratings in sequence. You'll see the depletion curve — and you'll see exactly where your high-focus window lives. This artifact becomes the basis for every scheduling decision you make going forward.