My report on the rise of night swimming for the Observer‘s ‘With nightclubs in decline, what does modern Britain get up to after dark?’ feature:

After night has fallen, in a secluded spot along the Bournemouth coastline, Susanne Masters and her friends swim with the sea fairies. This is their name for bioluminescence – light emitted by microscopic sea creatures so faint it disappears at even the glimmer of an electric lamp.

“It’s like sparkles that come off your body as you move through the water,” says Masters, who has been night-swimming there for around five years. “Occasionally we’ve found floating clumps of it. You can scoop your hand in the water and pull out a glowing thing the size of a 50p coin.”

The 38-year-old botanist and three close friends she met through a local outdoor swimming group often take sleeping bags to stay on the beach after their night-time dips. They fit these escapades around their everyday routines, she says, instead of living life “in increments of annual leave and weekends”.

Around the country, swimming under the stars has been quietly developing from an eccentric nocturnal trend into a legitimate evening pastime. Late-night swim sessions are springing up at lidos and open waters from Berkshire to northern Scotland.

“We’re not normally open at night, but we put glowsticks in the pool and have some mulled cider afterwards,” says James Totterdell, manager at Portishead Open Air Pool in Bristol, which held the first in its series of sold-out full-moon swims earlier this year. “It’s like an alternative night out, where you can be outside and active.”

But some, like Masters, prefer to organise their own impromptu night-swims. Colin Tallowin, a member of the Eastbourne Sea Swimmers, has kept a diary of every full moon outing the group has done since they had the idea two years ago. “The sea was inky silk, and the huge orange moon appeared low in the sky through the black clouds,” says one entry. Another describes how “seven lunatics” braved the choppy November currents to the disbelief of staff at a nearby fish and chip shop.

This new wave of night-swimmers is partly down to the rise in popularity of open-air swimming in recent years, says Kate Rew, founder of the Outdoor Swimming Society.

“Once people become more confident in the day, they start looking at different types of swims,” she says. “At night, what you hear and smell while swimming changes – as if you dim one sense, sight, the others are heightened.”

It’s important to know a place well in the daytime before taking the plunge at night, says Rew. She often swims in the Thames near Oxford, where it’s so quiet after dark you can hear twigs breaking under the water and night birds crashing into the trees.

“You can swim naked, with a swimming costume, or a wetsuit,” she says. “It depends on the time of year and how acclimatised you are.”

But while swimming at night could be associated with romantic skinny dips, an ice-cold bathe quickly stymies racy thoughts, says Masters.

“Swimming in cold English water is not very sexy. You go funny colours, and guys will come out looking like lobsters and then talk about how their manhood has disappeared,” she says.

“But there’s definitely a joie de vivre, and a bit of love and acceptance for your body and what it can do. The next day, it feels like you’ve been on an adventure.”