The optimal time depends on individual chronotype — the biological tendency toward morning or evening peak cognitive performance. Research consistently finds that most professionals experience their highest working memory capacity and lowest susceptibility to distraction in the first 2–3 hours after full waking alertness — typically mid-to-late morning for morning chronotypes. The practical recommendation: identify the 2-hour window in which you currently do your best thinking, and schedule deep work blocks consistently in that window, protecting it from meetings and shallow obligations.