Sleek, celeb-favoured gadget tracks sleep, activity and heart health without a smartwatch, but comes at high cost

Oura’s stylish smart ring worn by celebrities and athletes alike has slimmed down for its fourth iteration, making it easier to put on, more comfortable to wear and last longer between charges.

The Ring 4 swaps the clear plastic insides of its predecessor for shiny titanium to look even less like a cutting-edge piece of tech on your finger. It still weighs practically nothing – 3.3g to 5.2g depending on size – and comes in an expanded choice of 12 sizes and six finishes, including black, silver, gold and rose gold.