Accumulation of ‘toxins’ in brain is why thinking hard is tiring, study suggests

Researchers believe they have found out why thinking hard makes you tired (Kirsty O’Connor/PA)

A long, hard day of thinking can sometimes feel as exhausting as a day of physical labour, and now scientists say they know why.

Researchers suggest their findings indicate the reason people feel mentally exhausted, and not drowsy, from intense thinking is not just in their heads.