The renewed popularity of adult colouring books is unsurprising: they appeal at times of prolonged uncertainty, cognitive overload and emotional fatigue.

"Cozy coloring book" is a trend on BookTok, where users share videos of their favourite finished pages, collections of markers, and luxuriate in colouring as a Saturday morning or Sunday evening activity.

Colouring books were originally designed for adults, hundreds of years ago. Their renewed popularity now is unsurprising: they appeal at times of prolonged uncertainty, cognitive overload and emotional fatigue.

The last time adult colouring books trended was 2020, during the Covid pandemic - after their initial burst of popularity in 2015. Those 2015 bestsellers, like Secret Garden, which sold an estimated 2.2 million copies worldwide in one year, typically contained intricate patterns.

Ten years later, the "cosy" colouring bestsellers are much simpler, even childlike. Even darker books on the colouring bestseller lists - like the parody Cutest Serial Killers - are characterised as "cute", with simple illustrations.

These books favour simple, bold designs. Titles such as Cozy Corner focus on familiar domestic scenes -- reading nooks, kitchens, cafes -- while Cozy Cuties leans into playful animals, gardens and gentle, cheerful motifs. It's all rendered in broad outlines and large shapes that can be coloured with minimal...