Research by Cal Newport and the neurophysiology of ultradian cycles both converge on approximately 90 minutes as the optimal single session duration for most knowledge workers — long enough to enter and sustain a genuine focus state through the primary cognitive challenge, and short enough to maintain quality throughout without significant performance degradation. Newport notes that most professionals have a daily deep work capacity of 1–4 hours total. Starting with one 90-minute session per day and building toward two sessions as practice develops is the most reliable progression pathway.