Do you ever find yourself taking far longer to complete something than it normally should?
You read the same paragraph three times. You struggle to find the right words. Simple decisions suddenly feel unnecessarily difficult. Eventually you're just staring blankly at your computer wondering where your brain has gone.
Welcome to mental fatigue.
Mental fatigue is a normal response to prolonged periods of concentration and cognitive effort. Just as your physical performance can decline after doing lots of physical work, your ability to concentrate and perform mentally can suffer after spending hours thinking, problem-solving, making decisions and dealing with stress.
You can't expect to run as fast after running all day.
Likewise, you probably shouldn't expect your concentration, decision-making and patience to be exactly the same at 5pm as they were at 9am.
There isn't a magic supplement that fixes this, but there are a few things worth getting right.
HYDRATION
Water is essential for pretty much everything your body does, including normal brain function.
Your exact body-water percentage varies considerably depending on things like body composition, age and sex, so you don't need to worry about calculating exactly how many kilograms of you are water.
What matters is avoiding significant dehydration.
Dehydration can affect how you feel and perform, particularly when combined with things like exercise, heat or prolonged periods without drinking. Headaches, tiredness and difficulty concentrating can all accompany dehydration.
You also don't need to spend your entire day aggressively drinking water because you're frightened that thirst means you're already dehydrated.
For most healthy people, thirst is actually a useful part of the body's system for regulating fluid intake.
Keep drinks available throughout the day, drink regularly, and drink more when you're exercising, sweating heavily or somewhere hot.
Your urine can provide a rough additional guide. If it's consistently very dark and you're rarely going to the toilet, you may want to drink a bit more.
There's no prize for producing completely transparent urine all day either.
SLEEP
If you're constantly mentally exhausted, sleep is an obvious place to look.
Both the amount and quality of your sleep matter. One poor night's sleep doesn't mean your day is ruined, but regularly getting insufficient sleep can affect attention, reaction time, memory, mood and decision-making.
Unfortunately, you can't always control exactly how well you sleep.
You can, however, improve the conditions around it.
Some useful things to experiment with include:
Give yourself enough opportunity to sleep. Most adults generally need somewhere around 7–9 hours, although individual needs vary.
Keep a reasonably consistent schedule. In particular, a relatively consistent waking time can help regulate your sleep-wake cycle.
Give yourself time to wind down. That doesn't mean screens are inherently evil. If emails, work or doom-scrolling keep your brain switched on, however, putting them away before bed is probably sensible.
Pay attention to caffeine. Caffeine can remain in your system for hours. If your sleep isn't great, experiment with moving your last coffee earlier rather than assuming caffeine doesn't affect you because you can still fall asleep.
You don't need the perfect nighttime routine.
You need one that helps you sleep.
MOVE
If you've spent three hours staring at the same spreadsheet and your brain has turned to porridge, getting away from it for a few minutes can be surprisingly useful.
That doesn't mean sitting at a desk is slowly starving your brain of blood.
It's much simpler than that.
Breaking up prolonged periods of sitting with some movement is good for your health, and short bouts of physical activity can also help with alertness, mood and aspects of cognitive performance.
So get up.
Walk around the building. Go outside. Make a coffee. Take the stairs. Have a walk at lunchtime.
Five or ten minutes away from your desk isn't necessarily five or ten minutes of lost productivity if you come back considerably more capable of doing your job.
And if you can regularly exercise outside work, even better. Physical activity has benefits that extend well beyond your muscles.
TAKE A BREAK
This one probably deserves its own section.
Sometimes the answer to mental fatigue is not trying to find another way to force yourself through it.
It's stopping for a bit.
Sustained concentration is difficult. Short breaks can give you an opportunity to move, eat, drink, change your environment and return to a task with a little more attention.
That doesn't mean you need a complicated productivity system involving seventeen timers and an app.
If your productivity has fallen off a cliff, occasionally walking away from the task for ten minutes is more useful than spending another hour pretending to work.
FOOD AND CAFFEINE
Your brain requires energy, but you don't need special "brain foods" to get through the working day.
Regular meals containing a sensible mixture of protein, carbohydrate, fats, fruit and vegetables will do a perfectly good job for most people.
Caffeine can temporarily improve alertness and reduce feelings of fatigue, which is precisely why half the country starts its working day clutching a coffee.
Just remember that more isn't always better.
Using increasingly large amounts of caffeine to compensate for consistently poor sleep can eventually become a fairly rubbish cycle: tired → caffeine → worse sleep → more tired → more caffeine.
Use it as a tool, not life support.
WHAT ABOUT SUPPLEMENTS?
I wouldn't recommend fish oil as a treatment for mental fatigue or as a way of "increasing blood flow to the brain."
Omega-3 fatty acids are important nutrients and oily fish can absolutely form part of a healthy diet. But that's a different claim.
For most people struggling with everyday mental fatigue, supplements are going to sit a long way behind:
Sleep → breaks → movement → food → hydration → managing workload and stress.
The boring stuff wins again.
IN CLOSING
Mental fatigue is normal.
If you've spent the entire day concentrating, solving problems, dealing with people and making decisions, it isn't surprising that your brain doesn't feel as sharp by the end of it.
You can't remove every source of stress or cognitive demand from your life, but you can make sure you're not making things unnecessarily difficult for yourself.
Sleep enough. Eat properly. Stay hydrated. Move. Take breaks. Use caffeine sensibly.
And accept that occasionally the most productive thing you can do is stop staring at the bloody screen.
If fatigue or problems with concentration are persistent, severe, unexplained or significantly affecting everyday life, that's different from the normal end-of-day mental slump and is worth discussing with a healthcare professional.