Why your brain can make you feel tired even when you haven’t done much
Ever spent several hours sitting at your desk, scrolling through your phone, studying or working, only to feel completely exhausted.
You may not have done anything physically demanding, but your brain could still be experiencing mental fatigue.
Mental fatigue develops after prolonged periods of demanding cognitive activity, such as concentrating, making decisions, solving problems or constantly processing information.
A 2025 review published in Trends in Cognitive Sciences explains that prolonged cognitive work can affect brain systems involved in cognitive control and motivation.
As mental fatigue builds, tasks requiring effort can begin to feel more difficult, while the brain may increasingly favour activities that require less effort and offer quicker rewards.
This could help explain why, after a long day of studying or working, people often find themselves reaching for their phones, watching videos or doing other relatively effortless activities instead of starting another demanding task.
It is not necessarily a lack of discipline — the brain’s willingness to invest more effort can change when cognitive fatigue sets in.
Research has also found measurable changes in brain activity during mental fatigue.
A systematic review published in Psychophysiology analysed 21 studies involving healthy adults and found significant changes in EEG activity after mentally demanding tasks.
The important takeaway is that rest isn’t only necessary after physical exertion. Periods away from demanding cognitive tasks can also matter.
Taking breaks, getting adequate sleep, moving around and switching to less mentally demanding activities can help prevent prolonged cognitive strain.
So the next time you finish a day of studying, working or making countless small decisions and wonder why you’re exhausted despite barely moving, the answer may be simple: your body wasn’t running a marathon, but your brain was doing a lot of work.
