Dakota Johnson turned 37 on 4 October, and one of her sentences has been circulating longer than any of her films: that she could sleep 14 hours a day. She later clarified that she doesn't sleep that long every day, just that she "easily could". On Jimmy Fallon's show she joked: "I like to sleep, why is that bad?" She told WSJ Magazine that sleep is her "number one priority in life" and that she doesn't function on less than 10 hours.

The statement quickly became a "beauty secret". Skin experts are more sceptical. María Ángeles de Broto, a cosmetologist and founder of a skincare brand, confirms that skin really does work at night: "While we sleep, the skin enters a particularly important phase of renewal and repair." Night is when the skin restores its balance, and poor sleep, if it keeps happening, damages its protective barrier, and signs of fatigue and a lack of glow show up on the face.

But more doesn't mean better. Sleep, she says, doesn't work "in direct proportion", so extra hours don't guarantee extra benefit. The recommendation for adults is 7 to 9 hours a night, and "the quality and regularity of rest are just as important as the quantity". Her conclusion is clear: "Sleeping 14 hours may be a personal preference, but we wouldn't recommend it as a beauty treatment."

The flip side is far less pleasant. Dr Sofía Ruiz del Cueto, an aesthetic medicine expert, explains that insomnia brings excess cortisol and a drop in the biological signals for renewal. The changes in the skin, according to her, are similar to those caused by smoking: waste products are poorly cleared out, and fibres that don't renew build up.

So what actually helps? The experts recommend the 10-3-2-1 rule: no caffeine ten hours before bed, no food or alcohol three hours before, no work two hours before and no screens in the last hour. Dinner should be two to three hours before bedtime, and the phone farther away from the bed.

Johnson can sleep her ten hours. For most people whose alarm goes off at six, the real secret for their skin isn't 14 hours, but seven to nine, regularly, without scrolling until midnight. How many of us spend the last hour before bed with a phone in hand, and then go looking for a cream in the morning to fix it?