They frustrate you, boggle your mind, and make you sick of numbers altogether, but a daily Sudoku comes with its own reward for those who are prepared to stick with them.
According to a new study from the University of Exeter and King's College London, completing one of the brain-testing number puzzles every day can give midlifers the brain of someone 10 years younger.
The research had already found crosswords capable of the same brain-boosting effects, but this is the first time it has found Sudoku just as effective. Leader of the research, Dr Anne Corbett of the University Of Exeter Medical School, said: "We've found that the more regularly people engage with puzzles such as crosswords and Sudoku, the sharper their performance is across a range of tasks assessing memory, attention and reasoning.
"The improvements are particularly clear in the speed and accuracy of their performance. In some areas [it] was quite dramatic - on measures of problem-solving, people who regularly do these puzzles performed equivalent to an average of eight years younger compared to those who don't."
But puzzles aren't the only way to anti-age your brain...
Look after your physical health
There is significant research linking physical health and brain function. One study found that just half an hour of moderate exercise improved adults' brain-processing speed, while another found that weight-lifting increased the growth of nerve cells. And it's not just about exercise either. Preventing the build-up of dental plaque can reduce the risk of a stroke, according to some research, while another study found that high blood pressure can lead to reduced brain performance.
Learn a new language
Ted Mentele, editor in didactics at Babbel, explains that learning a new language activates the brain's neuroplasticity - it's ability to expand and form new neural connections.
"Language learning in adult life could improve your abilities in your elderly years. White matter, which is what allows you to better connect neural cells and improve your cognitive capacities, was actually increased in a group of adult students learning Chinese who were tested over a nine-month period."
Read more books
Scholars at the Fisher Center for Alzheimer's Research Foundation found that the mental stimulation provided by reading helps protect memory and thinking skills as we get older. The study's authors suggested reading every day helps reduce cognitive decline in later life.
And there could be an even more powerful way to activate your brain, according to Ben Hughes, Head of Content at Blinkist, an app which condenses whole books down into short 'blinks'.
"Learning in blinks gives you the information in short bursts, making it easier to commit to your memory," says Hughes.
Maintain your friendships
There's a reason why we tend to form close-knit bonds with those around us - it's good for our health, especially our brain health.
One study found that positive relationships with friends in old age can help stave off memory loss and other cognitive disorders. Memory decline among those without a wide social circle was twice as fast as among those with a lot of friends.
Perhaps it's because even simple interactions and basic conversation improves memory skills and sharpens the mind's ability to block out distractions, says one study.
And if you get really friendly with people? Even better. Sex increases brain function by boosting the level of serotonin and oxytocin in the brain. The former helps boost creativity and logical decision making, while the latter helps with problem-solving.
Eat more Marmite
You might love it, you might hate it, but either way, you should be eating it. In 2017, researchers from York University suggested that the high concentration of Vitamin B12 in Marmite increased levels of gamma-amino butyric acid (GABA), a neurotransmitter in the brain. The other great advantage of GABA is that it can help make people feel more relaxed, soothing anxiety, inducing sleep, and calming nerves.
And Marmite lasts a while. After participants consumed a teaspoon of Marmite a day for a month, the scientists noticed increased levels of GABA in their bodies for two months after that.
Jack Rear © The Telegraph
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