Energy management: why to manage energy, not time

Time management optimises a quantity that is fixed and identical for everybody. There are 168 hours in a week for a founder and for an intern, and the discipline is deciding which commitments get which of them. Everything involving other people runs on it.

Energy management optimises a different quantity: the capacity to think, which varies by hour of the day, by person, and by what you did in the preceding hour. The phrase entered mainstream management writing with Tony Schwartz and Catherine McCarthy's 2007 Harvard Business Review article Manage Your Energy, Not Your Time, which argued that organisations treat people as though hours were the only input, and that capacity, unlike time, can be renewed.

The two are not rivals. They answer different questions, and a schedule needs both.

The practical reading: time management decides what gets done this week, energy management decides which hour each thing gets. Do only the first and your best hours go to whatever was booked into them earliest, which is rarely your most valuable work.

C. Northcote Parkinson, a British naval historian, opened an essay in The Economist in 1955 with the line that became his law: work expands so as to fill the time available for its completion. His subject was bureaucratic growth, his evidence that the British Admiralty's administrative staff kept growing while the fleet shrank. The generalisation stuck because everyone recognises it from their own week.

Parkinson later added a second law about money, that expenditure rises to meet income. Both describe one mechanism: a resource with a boundary but no target gets consumed up to the boundary. A meeting scheduled for an hour uses the hour. Preparation with a free evening in front of it absorbs the evening.

Two things follow; only the first is usually taken away.

• Shorten the container. The standard corrective, and it works: size the deadline to what the task needs rather than to what is available. Done deliberately, this is timeboxing, covered in time blocking and timeboxing.
• Notice what the container is made of. Parkinson's Law is about elapsed time, but the thing consumed is attention. A three-hour task that expands into six does not cost three extra hours; it costs the capacity something else needed. This is where time management hands over to energy management.

The case that quietly costs the most is preparation, the one activity nobody bounds. Give it a container sized to the stakes (fifteen minutes for a status update, forty-five for a decision) and put that container outside your peak: prep is shoulder work.

Chronotype is the individual timing of your circadian rhythm: whether your body clock runs early, late, or in between relative to the clock on the wall. Research measures it with questionnaires, principally the Morningness-Eveningness Questionnaire published by Jim Horne and Olov Östberg in 1976 and the Munich Chronotype Questionnaire from Till Roenneberg's group.

Two findings there are solid enough to plan against: chronotype is substantially biological rather than a matter of willpower, and it shifts with age, running late through adolescence and early adulthood and drifting earlier after. A late type at a 9:00 design review is spending their worst hour, and no enthusiasm converts it into their best one.

The framework most people meet is not the questionnaire but the four animals from Michael Breus's 2016 book The Power of When, which keeps the folk distinction between larks and owls, renames it, and adds two more types.

Two caveats. The four-animal scheme is a popular framework rather than settled science: in the research literature chronotype is a continuous distribution, not four categories, and the population splits Breus attaches to the animals are his own estimates. Treat it as vocabulary for a real phenomenon, not as a diagnosis.

Second, chronotype is only one input. Sleep debt, caffeine timing, illness, small children and a long commute all move the curve, in some weeks more than biology does. Which argues for measuring your own curve rather than reading it off a table.

Circadian rhythm explains the shape of your day. Ultradian rhythms are the shorter cycles nested inside it. The idea traces to the sleep researcher Nathaniel Kleitman, who identified a roughly 90-minute cycle in sleep stages and proposed that a version of it (he called it the basic rest-activity cycle) continues through waking hours, alternating higher alertness with periods where sustained attention degrades.

State this carefully. The 90-minute cycle in sleep is thoroughly established; the waking version is much less firmly pinned down, and the popular claim that focus runs on a precise 90-minute clock is a tidier story than the evidence supports. What is not in dispute is the direction: sustained cognitive effort degrades over time, the degradation is not linear, and a break restores more than continuing does.

So the useful form is a range, not a number: expect 60 to 120 minutes of genuinely good focus, then diminishing returns steep enough that pushing through costs more than it produces. Three consequences:

• Size deep work blocks at roughly 90 minutes, not at four hours. A four-hour block is one good cycle followed by three hours of decreasingly useful company.
• Take the break at the end of the cycle, not at the low point. A break at minute 95 is recovery; the same break at minute 25 is an interruption.
• Do not stack three demanding meetings. The third lands in a trough the first two created, and it is usually the one where a decision was due.

The Pomodoro Technique's 25-minute interval is not a rival theory but the same insight at a smaller scale: a starting ritual for work you are avoiding, which composes with the longer cycle rather than replacing it. See Pomodoro and Atomic Habits for when the short interval helps and when it fragments work that needed a run at it.

Once you have a peak and a trough, scheduling becomes a sorting problem. Some work requires generating something that did not exist, some requires following a structure that already does, and some requires only that you are present and awake. Those three have very different energy prices.

The trough row is the one people resist, because leaving a mid-afternoon hour for expenses feels like waste. It is the opposite. That hour will be bad for thinking whether or not you schedule something demanding into it; the only choice is what you spend it on.

Three rules make this survive a real calendar. Start with the commitments you own, because a weekly 1:1 moved once stays moved and an all-hands does not. When someone else owns the invite and it lands in your trough, the lever is the preparation rather than the time: arriving with the thinking already done turns generation into presentation, which is shoulder work. And put the peak on the calendar as a titled block, because an unlabelled empty hour reads as availability.

Prioritisation and energy matching are separate steps that get conflated. Deciding what matters is a job for the Eisenhower Matrix and the Pareto principle; deciding which hour it gets is this. Do the second without the first and you spend your best hour on an urgent triviality, very efficiently.

A late type with an early standup

An engineer whose log shows 2s until about 11:00, a strong block from 11:00 to 13:30, a dip, and a second good window after 16:00. The 9:15 standup is not moving. Rather than fight it, they use 9:30 to 11:00 for code review, triage and replies, all shoulder work with a structure already in place. The 11:00 to 13:00 window becomes a calendar block titled with the actual task, and pairing gets proposed for after 16:00 instead of the 14:00 it had drifted to. The team's schedule did not change; two hours a day changed price.

A manager whose calendar is not their own

A manager with roughly a dozen meetings a week and a peak between 8:30 and 10:30. Almost none of the meetings are theirs to move, so they work on the two things that are. The four 1:1s they own come out of the morning into the 14:00 to 16:00 band, a good use of a mediocre hour because a 1:1 has structure and a second person supplying half the energy. And prep gets a fixed twenty-minute box the afternoon before, which is Parkinson's Law used deliberately, freeing the morning for the one block of hard writing. The meeting count did not fall. The week stopped charging its best hours for them.

A consultant with a real post-lunch dip

The early-afternoon dip is not caused by lunch; it appears in the research as a circadian feature, and a lighter lunch reduces it rather than removing it. A consultant with a reliable 13:00 to 15:00 trough stops treating it as a failure of discipline and gives it a job: client calls that need presence rather than invention, plus expenses and timesheets. A ten-minute walk at the bottom of the dip does more than a fourth coffee, which mostly moves the debt into the evening. The demanding client work moved to 9:30. Same hours, different result.

If capacity is what you are managing, restoring it is part of the work. Schwartz and McCarthy's argument was explicitly about renewal rituals, and the practical version is narrow.

• Ten minutes away from a screen beats ten minutes of email. A break spent in your inbox is not recovery; it is a shorter, worse meeting.
• Movement does most of the work. Standing, walking to another room, going outside. The reset is physiological, which is why it works when you do not feel like it.
• Close the previous task before starting the next. Sophie Leroy's work on attention residue found that switching away from an unfinished task leaves part of your attention on it, degrading performance on the new one. Writing down the decision and the next step ends the task; intending to remember it does not.
• Guard the boundary of the day. The evening rebuilds tomorrow's capacity and is the first thing sacrificed when a deadline slips. Chronic borrowing from it flattens the curve until there is no peak at all, which no scheduling can fix. Digital minimalism covers the attention side of that boundary.

One corollary: the deepest concentration cannot be scheduled into a trough. Flow needs a challenge matched to your skill and uninterrupted runway to get into it, both peak-hour conditions. Flow, deep work and the three enemies of focus covers them; energy management is what puts you in front of the work while you can still meet them.