When we are doing hard cognitive work people get tired after 5-6 hr. of hard thinking work, but if we do some other kind of work where thinking not involved people feel less tired,that means mentally hard task makes you tired.
Hard cognitive work (such as thinking, reasoning, or remembering)
leads to glutamate accumulation in the lateral prefrontal cortex of the brain. this research published in The current biology journal
Glutamate regulation reduces the control exerted over decision-making
High-demand cognitive work resulted in higher glutamate concentration
and glutamate/glutamine diffusion in a cognitive control
brain region (lateral prefrontal cortex
Feeling of tiredness is the Brains new way telling you, stop working so that glutamate level get normal.
Taking rest, sleep or break from your work is the only way by which we can restore brains working normal.
after all we are not machine so we need rest after hard cognitive work.