Most productive time of day: find your peak hours

Tom sits down at nine in the morning to a half-written text and an hour later he has finished the thing that usually costs him half an afternoon. At three o'clock the same man reads one email for the fifth time and still cannot work out what the sender wants from him. Same brain, same desk, six hours of difference.

We measure working time in hours and behave as if the hours were interchangeable. Eight hours of work sounds like eight identical slices. Your body clock sees it differently: across the day your core temperature moves, so does your cortisol level, your reaction speed and your capacity to hold several things in mind at once. Performance therefore follows a curve rather than a straight line. Lay a flat calendar over that curve and part of your best thinking goes into a meeting about the budget.

Every day has a peak, a slump and a second wind

For most people the day breaks into three distinct parts. The first is the peak, which arrives roughly two to four hours after waking and holds for a few hours. Body temperature is rising, attention is sharp, working memory carries more at once. This is when logic, numbers and putting thoughts into words come easily.

Then comes the slump. Typically six to eight hours after waking, for most working people somewhere between one and four in the afternoon. Alertness drops, errors multiply, reading the same paragraph takes twice as long. It is not simply the effect of lunch, and it is not about food at all: the slump turns up in people who skip lunch entirely.

The third phase is the second wind. A late-afternoon lift, when energy climbs a little again before the evening pulls it down for good under mounting sleep pressure and the release of the hormone melatonin. In some people the second wind is unmistakable, in others barely noticeable.

Two forces produce all of this. The first is sleep pressure, which starts building the moment you get up and grows more or less steadily all day. The second is the alerting signal from your internal clock, which rises and falls and lifts again at a certain point in the day. The afternoon slump appears where sleep pressure is already high and the circadian signal has not yet climbed back up. The early-evening second wind is that signal's later wave, which overrides the pressure for a couple of hours. It is why you can feel sharper at six in the evening than at two in the afternoon, despite being four hours more tired.

That this is not a subjective impression was shown in 2011 by the sociologists Scott Golder and Michael Macy of Cornell University. They analysed hundreds of millions of tweets from millions of users across dozens of countries and tracked how the emotional tone shifted through the day. The curve that came out was surprisingly stable: mood high after waking, a decline through the day, a mild recovery towards evening. More striking still, the same shape repeated across cultures and continents, among English-speaking users in North America and in Asia. The daily rhythm of mood and alertness is unlikely to be manufactured by office culture, then. It is manufactured by biology we hold in common.

What belongs in the peak and what survives the slump

Once you know when your curve rises, the logic of planning changes. The peak belongs to work that demands full brain capacity: focused, uninterrupted blocks of the kind now commonly called deep work. Drafting a proposal, analysing data, building an argument, a difficult conversation, a decision that is hard to reverse. Do them in the best hour of your day and you save yourself two hours of rewriting.

The slump, on the other hand, is the sensible home for anything that runs on half your attention. Clearing email, record keeping, tidying up files, the trip to the post office, the shopping, routine calls. A brain that cannot currently hold a complex structure can still tick items off a list.

Meetings are a chapter of their own. Most companies pile them into the morning, which is precisely the stretch when a large share of people have their sharpest heads. An hour of the weekly status meeting at ten in the morning costs far more than an hour of the same meeting at two in the afternoon, even though it occupies the same square in the calendar. If you have any say over when your team gathers, moving recurring meetings into the slump is among the cheapest changes with an immediately visible effect.

There is one more option that tends to be forgotten: the slump can be spent moving. Twenty minutes of walking at half past two does more for the afternoon curve than a third coffee. Why the afternoon dip happens and what makes it worse we cover in a separate piece on the afternoon slump.

Same curve, different position on the clock

Here comes the crucial qualification. The shape of the curve is shared, its placement on the clock face is not. That placement is what chronotype describes: the tendency of your internal clock to run earlier or later than the one on the wall. Between a strongly morning type and a strongly evening type the peak can sit four or five hours apart.

The morning type, the lark, wakes without help and thinks best before noon. The dove, the middle type that covers the largest share of the population, works a little later and has a flatter day. The evening type, the owl, gets going slowly and tends to be sharpest around the time everyone else goes home.

A second property sits alongside your position on the morning-to-evening axis, and it is routinely overlooked: how stable your rhythm is. Some people have a steady rhythm and their curve looks much the same on a Tuesday as on a Sunday. A change of regime, a night shift or a flight across two time zones throws such a person off for several days. Others have a flexible rhythm, adapt faster, and in exchange slide more easily into irregularity, where the peak smears out and stops being something you can rely on.

The practical consequence: two owls may need completely different working arrangements. An owl with a steady rhythm gets by with shifted hours, because she knows she will still be precise at ten in the evening. An owl with a flexible rhythm needs fixed anchors instead, otherwise the day drifts an hour somewhere else every week.

Chronotype is not a lifetime stamp either. It shifts with age: small children are markedly morning types, in puberty the internal clock slides backwards, a person is at their latest at around twenty, after which it slowly travels forward again. For the mismatch between when the body wants to sleep and when the alarm allows it, the chronobiologist Till Roenneberg uses the phrase social jet lag. It describes living in one time zone during the working week and a different one at the weekend, without having flown anywhere. And the bigger the gap between the hour you wake on Tuesday and the hour you wake on Sunday, the harder your own peak is to find.

Ideas turn up at the wrong hour

An easy rule suggests itself: cram everything important into the peak. Except that research adds something unexpected here.

In 2011 the psychologists Mareike Wieth and Rose Zacks gave students two kinds of task. Analytical ones, where you reach the result step by step, and so-called insight problems, where the answer either arrives whole or does not arrive at all. Some people worked at their best time of day, others outside it. On the analytical tasks the outcome was as expected, with no great difference. On the insight problems people did better away from their peak. Morning types had more insights in the evening, evening types in the morning.

The explanation makes sense the moment you hear it. At your peak the brain filters well. It stays on topic, rejects distracting associations, drives straight at the goal. That is exactly the advantage in analysis and the handicap in creative work, because an idea usually comes from connecting two things that look unrelated at first glance. A more tired brain filters more loosely, lets in more side thoughts, and occasionally the right one is among them.

Hence a division of labour that looks counterintuitive: analysis and execution in the peak, brainstorming and hunting for new directions perfectly fine in the slump. If you have ever had your best idea in the shower or on the way home, that was not chance but a loosened mind in an hour when proper work was no longer possible.

Finding your own peak without a laboratory

Most people estimate their best time of day from when they have the most work, not from when they think most clearly. A week of honest note-taking exposes the gap. No app required, a notebook or a memo on your phone will do, plus one box per hour.

What to watch for over the week:

• "How is my concentration right now on a scale of 1 to 5?" Write the number down and do not spend more than a couple of seconds on it.
• When you first reached for coffee today, and above all why. A ritual at nine is a completely different animal from an emergency brake at two in the afternoon.
• The moment you stopped writing and started scrolling. That is usually where the slump begins, whether or not you admit it.
• The moment something occurred to you that had nothing to do with the task in hand. Ideas run to a timetable of their own, which need not agree with the peak.
• Wake-up times without an alarm. A weekend and a holiday give away more about your internal clock than an entire working week.

After seven days, write the data out underneath each other and look for repetition. It is not about one exceptional day but about the hours that come out well or badly on a regular basis. For most people the pattern is already unmistakable after five days.

Two warnings, so the week does not lead you astray. First, a badly slept night drags the whole curve down and the day after says almost nothing about your chronotype, so mark those days and drop them when you review. Second, a strong dose of caffeine can mask the slump for an hour or two. If you drink coffee steadily all day, watch the moments when you reached for it rather than the moments when you felt tired.

And now a question worth genuinely answering rather than skimming: what time were you at your sharpest today? If the answer did not appear within a moment, that is useful information in itself. It means you plan your day around other people's demands rather than your own capacity. If you have no idea where on the lark, dove and owl axis you belong, a short chronotype test gives you a rough estimate in a few minutes, and a week of notes will then confirm or refine it.

Plan around the curve, not the calendar

The calendar is the worst adviser you have on this question. It shows free windows, not your capacity. An empty Tuesday afternoon looks like the ideal slot for a demanding analysis, but if your head is dropping at half past two, you are asking for two hours of pretend work.

The reverse procedure is simple. First draw your curve on paper. Then place one or two things a week that genuinely matter into its upper section, and only after that arrange the meetings, the admin and everything else around them. Meetings will almost always accommodate you. Your biology will not.

Reality resists, of course. An owl on a shift starting at seven in the morning has no way to move her peak, and that is a finding in itself: if your work runs permanently against your internal clock, you pay for it in a tiredness you explain to yourself as a lack of discipline. Even a fixed timetable, though, will usually yield half an hour. Defend one block against meetings, shove the morning stand-up into the stretch when you are useless anyway, or negotiate two days a week with a later start. How the internal clock, sleep and performance hang together we take apart in more detail in the overview of what a chronotype is and how to work with it.

Tom from the opening did nothing dramatic in the end. He moved his morning email check from nine to eleven and put writing into the window that opened up. The afternoon hour in which he used to read one message five times now goes on correspondence and a short walk. The volume of work stayed the same, so did the number of hours worked. Only the order changed.

Try one concrete thing. Open your calendar for next week and find two hours that fall inside your peak and are still empty. Block them before somebody else claims them, and write into them the name of the task you have been avoiding longest. You will see how differently it goes when you let it at your brain in the hour when there is most of it.