Sixteen people pedalled an exercise bike until they had to stop. On one visit they first spent 90 minutes watching documentaries. On the other they spent 90 minutes on a demanding mental task. After the thinking they lasted 640 seconds, against 754 after the films: nearly two minutes less. Their hearts, lungs and muscles responded much the same way on both visits. The pedalling felt harder, so they reached their limit sooner. It's one reason a day spent sitting can leave you too tired to walk.
Energy gets described as a tank: full in the morning, empty by evening, topped up with food and coffee. The picture has a pedigree. In the 1920s A. V. Hill, fresh from a Nobel Prize, ran experiments in London that put fatigue in the muscles, and the idea held for most of a century.
Those sixteen pairs of legs were the same legs on both visits. Tiredness behaves more like a gauge, a reading your brain produces, and a gauge can be pushed up or down. Ahead: what a sugar drink did for cyclists who spat it out, the rigged race that made nine riders faster, how being told you slept well changes your score on a test, what a face you never saw does to your stamina, and what a day of hard thinking leaves behind in the brain.
1. Put the body before the spreadsheet
Hard thinking makes the next physical effort cost more, and the observation is more than a century older than that bike. In Turin in 1890, the physiologist Angelo Mosso had built a machine he called the ergograph, a "register of work". It held the hand still and recorded how many times the middle finger could lift a weight, once every two seconds, before it gave out. His colleague Maggiora, a lecturer in hygiene, sat down at it on the morning of the university's oral exams and managed 40 lifts. After questioning fourteen candidates he came back and managed 24.
Mosso liked a machine. He also built a balance to detect blood shifting towards the brain while a person was thinking, now regarded as the first brain-imaging technique.
Later tests agree. Across 11 sets of findings, long mental work cut endurance and made effort feel harder, while heart rate, oxygen use and the other bodily measures stayed put. Maximum strength and power were untouched. A tired head costs you the long effort and leaves the short, heavy one alone.
Some heads resist better. Half an hour of a hard mental task cut the power nine recreational cyclists could hold for a 20-minute ride from 226 watts to 216. Eleven professionals held theirs: 326 after an easy task, 323 after the hard one. Whether they were born that way or built it over years in the saddle, nobody can yet say.
Roger Bannister, the first person to run a mile in under four minutes, put it in a line: "It is the brain not the heart or lungs, that is the critical organ."
So the run that feels impossible after a long day at a desk is the same run. The day has raised its price.
Do your walk or run before the hardest thinking of the day. If training has to come after the desk, make it the short, heavy session.
2. Count the phone as work
Scrolling tires the same gauge. Sixteen adults did three sets of squats, each to the point of failure, after half an hour on social apps, and on another day after half an hour of a documentary. After the scrolling they felt more mentally tired and got through less work. Sets to failure test how long you can keep going, which is the effort a tired head cuts short. Their blood and the way they moved the bar were no different.
Leave the phone in your bag for the half hour before you train. If it keeps finding its way back into your hand, a holds it until the time is up.
3. Know the signal from the fuel
Eight trained cyclists rinsed their mouths with a sugar drink for about ten seconds and spat it into a bowl. Their timed ride took about an hour, and they finished it more than a minute faster than when they rinsed with a drink sweetened to taste the same and holding no sugar. A second group did the same with maltodextrin, a starch, and finished two minutes faster.
On a brain scan, sugar in the mouth switched on reward areas that the sweetener left quiet. No fuel had reached a muscle. The mouth had told the brain that fuel was on its way, and the brain let more effort through.
Swallowing the drink is another matter. When sugar and other carbs were tested against dummy drinks, they gave mood no lift at any point, and within the hour people felt more tired and less alert.
Nine more cyclists showed the same reserve from another direction. Each raced an on-screen rider he'd been told was a replay of his own earlier ride over 4,000 metres. In one race it was. In another it had been set 2% more powerful without his knowing. They finished faster against the rigged rider than against the true one. The extra had been there all along, held back.
William James had described that reserve a century earlier, in an essay on what runners call second wind: "The fatigue gets worse up to a certain critical point, when gradually or suddenly it passes away, and we are fresher than before."
For the book-length version, Alex Hutchinson's follows the people testing where the limit sits.
The next time you want to cut a walk short, pick a lamp post further on and stop there instead.
4. See coffee for what it does
Caffeine works on the gauge. Across 21 sets of findings, it made the same exercise feel 5.6% easier while people were doing it. Nothing went into the tank. The reading was turned down.
Part of the lift is belief. Six trained cyclists were told they would get a dummy capsule, a moderate amount of caffeine or a large one on different rides. Every capsule was a dummy. They produced 1.4% less power when they thought they'd had the dummy, 1.3% more on the "moderate" capsule and 3.1% more on the "large" one. Six riders is a small group, so take it as a pointer.
The bill arrives at night. A strong hit of caffeine taken six hours before bed still cut people's sleep by more than an hour, and they were unaware of it.
The morning cup has its own catch. In daily drinkers, it mostly undoes the tiredness and low mood that a night without caffeine has built.
An energy drink is the same trade in a can: caffeine, usually with sugar.
Drop the afternoon coffee first, and watch what a week does to your evenings and your mornings. If it's the cup you'd miss, is roasted chicory with no caffeine in it.
5. Keep hunger out of it
Then there's hunger. When 119 overweight adults were put on a low-carb, ketogenic diet or a low-fat one, most of the symptoms they tracked eased on both. Two eased more on the low-carb diet: hunger and low mood.
Constant hunger is tiring company, and that's the part the low-carb plate eased most.
Hunger is a reading as well. Forty-six people drank the same 380-calorie milkshake on two days. On one it was labelled an "indulgent" shake of 620 calories, on the other a "sensible" one of 140. After the "indulgent" shake, ghrelin, the hormone that says "eat", fell steeply. After the "sensible" one it barely moved. How full they felt followed the label.
If hunger is what drains your afternoons, build your meals around meat, fish and eggs and cut the starch, whether you call it low-carb, keto or carnivore. Then sit down and eat it as the full meal it is.
6. Go easy on the verdict
What you're told about your night counts as well as the night itself. In two experiments, 164 people said how well they'd slept, then were told, at random, that their night had been above average or below it. The ones told they'd slept well did better on tests of attention and of finding words. How they said they'd slept made no difference to the scores. What they'd been told did.
It held for two tests out of four, and no verdict replaces a real night. It still shows how much weight the morning's verdict carries.
A thermometer can hand down a verdict too. Seven men cycled for half an hour in a room at about 31°C (88°F) and covered less ground than in a cool one: 15.9 kilometres against 16.6. On another hot day the display told them the room was 26°C (79°F) and showed their body temperature a little lower than it was. They covered 16.7.
After a rough night, skip the announcement that the day is ruined, and leave the sleep tracker's score unread until evening.
7. Drop the willpower tank
For years the textbook picture was that self-control drains like a battery: resist the biscuit now, and there's less left for later. Then 23 labs ran the classic test again with 2,141 people between them, and the effect all but disappeared.
What you believe turns out to matter. People who saw willpower as unlimited held steady after a tiring task, and in demanding weeks they ate better and put things off less than people who saw it as a tank.
A reason matters too. People worn down by one test of self-control did better on the next when they'd been told their effort would help others, or pay off for them, than worn-down people given no reason. It would explain the evening when you're too tired to tidy the kitchen and wide awake for the thing you love.
Notice the next time you say "I've got nothing left", and name who the next small job is for.
8. Talk to yourself like a coach
Twenty-four people rode to exhaustion, and one group then spent two weeks practising short encouraging phrases to say to themselves while riding. On the second ride that group lasted 750 seconds, up from 637. The others stayed where they were. Halfway through, the ones talking to themselves rated the effort lower.
Paavo Nurmi, who won nine Olympic gold medals at distance running, had his own version: "Mind is everything; muscle, pieces of rubber. All that I am, I am because of my mind."
Choose one short phrase for the hardest stretch of your day, and say it when the stretch arrives.
9. Set the mood before the effort
Thirteen people cycled to exhaustion twice while a screen flashed faces at them for 16 thousandths of a second, too brief to notice. With happy faces they lasted 1,519 seconds. With sad ones they lasted 1,342, about three minutes less, and rated the effort higher. None of them knew which faces they'd been shown.
Music does the same job in plain sight. Across 139 sets of findings and 3,599 people, music during exercise lifted mood, improved performance and lowered how hard the effort felt. Fast music did more for performance than slow.
Company does it too. Rowers from a college crew trained in time with each other on some days and did a similar session alone on others. After the sessions together, they could stand more pain.
Put on something fast before the job you've been putting off, or do it alongside someone.
10. Move when you're tired
Thirty-six healthy, inactive young adults who felt tired all the time were split three ways: easy exercise, moderate exercise, or none. The exercisers came in three times a week for six weeks. Feelings of energy rose as much with the easy sessions as with the moderate ones, and the easy sessions did more to ease tiredness. None of it depended on how much fitter anyone got.
Mosso had noticed it on his walks. "After an hour's exercise we are more lively; our limbs become loosened, our steps freer," he wrote, and he counted it among the body's marvels "that action does not depress and exhaust its energy, but renders it more fit for work."
On a flat day, swap your session for its easy version and still go.
11. Finish the shower cold
In the Netherlands, 3,018 adults either showered as usual or ended every shower with a burst of cold water for a month. Over three months, the cold-shower groups took 29% fewer days off sick. They reported just as many days of feeling ill. One reading: they felt as rough as everyone else, and went to work anyway.
The benefit they mentioned most was more energy, and a good number compared it to caffeine. Nine in ten said they wanted to carry on, and 64% did.
Cold is a jolt to the nervous system. Young men who spent an hour up to the neck in water at 14°C (57°F) had 530% more noradrenaline, the body's alerting chemical, in their blood, and 250% more dopamine. An hour in a cold bath is far more than a burst at the end of a shower. It does show which lever the cold pulls.
Only people without serious illness were tested. If you have a heart condition, leave this one out.
End tomorrow's shower with a short cold burst, and see how the next hour feels.
12. Breathe out slowly
Breathing exercises get sold as an oxygen top-up. What has been measured is calm. About a hundred people spent a few minutes a day for a month on meditation or on one of three breathing exercises. All four lifted mood and eased anxiety. The breathing did more for good mood than the meditation, and the exercise built on long, slow out-breaths did best, and brought people's breathing rate down as well.
Nobody in that test measured energy. Calm is still worth having when you're wound up and worn out at once.
Take a few breaths with the out-breath longer than the in-breath, and notice your shoulders.
13. Respect the real limit
None of this makes tiredness imaginary. Mosso saw the point of it in 1891. Fatigue, he wrote, "at first sight might appear an imperfection of our body" and "is on the contrary one of its most marvellous perfections". It rises faster than the work does, and stops us before we do ourselves harm.
Tiredness is one signal. Chest pain, dizziness or a sharp pain in a joint are different ones, and they mean stop.
Most of these tests used small groups of fit volunteers, so the size of the effect will differ for you. The direction is what carries over.
Hard thinking leaves a real trace as well. Two groups spent a working day on mental tasks, hard ones for one group and easy ones for the other. By the end, the hard-task group had more glutamate, a signalling chemical, built up in the part of the brain that keeps you on track. Their choices had shifted too, towards whatever was quick and easy.
Make the decisions that matter early in the day, and settle what's for dinner before the day wears you down.
14. Take your head outside
A spent head recovers faster in some places than others. People's attention, measured by how many digits they could hold and repeat backwards, improved after a walk among trees. A walk along busy streets left it where it was. Pictures of nature helped too, against pictures of the city.
A street asks for attention at every kerb, and a wood asks for next to nothing.
After a day of hard thinking, walk somewhere green before you sit down to anything else.
When it's more than tiredness
Everything above is for ordinary tiredness. If yours has gone on for weeks with no cause you can name, look for one.
Some causes are plain enough: pregnancy, the menopause, getting over an illness such as glandular fever, and some medicines. Others leave clues. Loud snoring with gasping or choking at night points to sleep apnoea, where breathing keeps stopping during sleep. Pale skin, breathlessness and a heartbeat you can feel point to low iron. Great thirst and frequent trips to the toilet point to diabetes. Tiredness that comes with restlessness, nerves and irritability points to an overactive thyroid.
Nutrient Deficiencies: Who's at Risk and Which Test to Get covers which blood tests to ask for.
Tiredness can also be part of low mood. If you've felt flat for weeks and stopped enjoying things you used to, reach out for help today.
Work can do it too. The World Health Organization describes burnout by three signs: exhaustion, a growing distance from your job, and feeling less effective at it. It ties burnout to work stress that has gone on unmanaged, and says it is "not classified as a medical condition". The fix there lies in the work, and no cold shower reaches it.
One condition turns all of this upside down. In ME/CFS, also called chronic fatigue syndrome, symptoms get worse after physical or mental effort and can take weeks to settle. "Start anyway" is the wrong rule there. Get it checked.
A gauge you can read
English has two words for it, and they hold the two theories. "Tire" goes back to the Old English tēorian, to fail, to cease: the tank running dry. "Fatigue" comes from the Latin fatigare, to weary, to vex, to harass: the gauge nagging you. A harasser deserves a hearing, and now and then an argument.
Tiredness is your brain's estimate of what the next effort will cost. A long day of thinking raises the price, and so does a bad verdict on last night.
Sleep and food are still the fuel, and no phrase replaces them. What you can do is stop taking the first "I'm done" as the last word.
The sixteen cyclists brought the same legs to both visits. After the thinking, the gauge called time 114 seconds early.
Tomorrow, move first and think second, and see how the afternoon feels.
Flagging after lunch? 5 Ways to Harness Your Energy and Beat the Afternoon Slump — the body's side of energy: sleep, a ten-minute walk, steady meals and water.
Ready to drop more than the afternoon cup? How to Quit Coffee: Getting Through Caffeine Withdrawal — what withdrawal feels like, and how to step down without the headache.
Know someone who comes home from the desk too tired for the walk they'd planned? Send them this, and offer to go along.
Disclaimer: This article is general information. For advice about your own health, talk to a qualified health professional. If your mood has been low for weeks or you're having thoughts of harming yourself, reach out for help today.
Affiliate Disclosure: This article contains affiliate links. As an Amazon Associate, All For Life earns from qualifying purchases, at no extra cost to you. We only link to products we would recommend without the commission.
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