For many, Christmas is a time to relax and overindulge. But, for some, it's an opportunity to take part in more vigorous activities than dozing on the sofa or enjoying a Champagne breakfast.
Hundreds of swimmers will be stepping into the sea on Christmas Day, while runners pound the pavement and cyclists attempt to find time between family commitments to ride hundreds of kilometres.
Why do they do it? And what do their loved ones make of the fact they are more likely to dress up in lycra than a Santa hat?
Sarah Hill, Surrey
Sarah Hill is a self-confessed exercise "addict", who takes a rare day off on 25 December. So, come Boxing Day, she is eager to get going again.
The 45-year-old, who lives in Farnham, said the 3.5-mile run through Devil's Punch Bowl, a nature reserve in Surrey, has become a part of her family's "festive ritual", with her two daughters, aged 13 and 15, joining spectators.
The former soldier and police officer, who recently retired after 22 years' service with Hampshire Constabulary, having been diagnosed with post traumatic stress disorder, said "running has been my saviour".
"You feel much more positive after getting up on Boxing Day and doing this," she said.
Far from opting to exercise in place of indulging, she said: "I love food, but I run so I can eat."
Organisers of the Boxing Day run encourage participants to drink a pint of ale at the mid-way "beer stop".
"The men all stand there like teapots, with their hands on their hips in a gaggle, like it was a Saturday evening," she said.
Ms Hill said she would try to swiftly finish the beer herself.
"You are monitored by a drinking judge, but you can't help to have some dribbling down your chin.
"Then you set off to run again and all you can hear is people heaving, wanting to throw up.
"It's disgusting, but a right laugh."
Val Smith, Liverpool
Val Smith, 55, will be clambering into Liverpool's Salthouse Dock on Boxing Day, before wading into the sea at New Brighton on New Year's Day.
She is one of a hardcore group of open water swimmers who brave the sea year-round, without the aid of a wetsuit.
"It's so important for me to get my dips in, especially over Christmas," she said. "I don't feel myself if I don't manage to."
"I live with my mum, who has Alzheimer's, so open water swimming is my stress relief."
"It makes a massive difference to how I feel. Open water swimming at this time of year - at any time really - makes me energised and relaxed."
The grandmother-of-two said she began open water swimming a decade ago wearing a wetsuit, but, when it broke, ventured out in only a swimsuit and hasn't looked back since.
She said the New Year's Day dip was the best, adding: "About 30 of us gather and charge into the water holding hands. It's become a bit of a spectacle and people come to watch.
"They think we're mad and you do get [people] asking about the cold a lot."
She said there was "no denying it's very cold," but added that "the more you do it, the more your body adjusts and you can control its response and your breathing".
She said the swims gave her a sense of euphoria, adding: "One year I went for a swim a week before Christmas in the River Dee in Chester and then went Christmas shopping in Chester afterwards. I was walking around on a high singing carols and people were looking at me."
Cath Bullock, Redcar
Cath Bullock will be joining scores of hardy souls, many in fancy dress, braving the North Sea in her hometown of Redcar on Boxing Day.
The event, organised by the local rotary club, attracts crowds of spectators and, according to the RNLI, appears to be increasing in popularity.
In previous years, the 64-year-old grandmother was among those watching from the relative warmth of the shore, but, having been "dared" by her daughter-in-law, she is preparing to take the plunge.
"I've thought about doing it for a few years," she said.
"Then my daughter-in-law who is visiting from Switzerland said 'if you do it, I will'.
"But the rest of the family think I've gone loopy."
The Rotary Club of Redcar said 278 swimmers took part in 2017, with similar numbers expected this year.
Dan King, Sussex
Dan King, from Hurstpierpoint in West Sussex, is one of thousands of cyclists who will be embarking on an ambitious attempt to ride at least 500km (310 miles) between Christmas Eve and New Year's Eve.
The distance should be no obstacle for the 33-year-old amateur racing cyclist, who rides an average of 250km a week, but finding time between family commitments can be more challenging.
He said his main concern was "how can I fit this in without annoying everyone too much?".
Signing up to the "festive 500" challenge, which was attempted by more than 11,300 cyclists in the UK last year, gives him a ready-made "excuse", allowing him to spend more time indulging his passion.
"It's almost like a job. You can say 'I need to go for a ride' and it gives you an answer when people say 'why are you doing that?'."
Mr King, a digital consultant, said his immediate family had grown accustomed to his cycling-induced absences. "It's got to a stage where it's just a given, but it is weird when you go somewhere with more distant family and they say: 'You've done what? You've ridden how far?'"
In past years, he has tried to complete a sizeable portion of the challenge on Christmas Eve. "I'll aim to get 200km under my belt, then if family stuff becomes an issue I can drop it and just do a few short rides."
He said the long hours in the saddle fuelled greater Christmas indulgence. "I get out in the morning, get the miles in, and then you can stuff your face all day and you don't even need to worry about it."
Katy Netherclift, Kent
Katy Netherclift, from Crockham Hill near Edenbridge, is aiming to ride at least 100km on Christmas Eve, as she takes on the "festive 500" challenge for the third year.
The 48-year-old self-employed gardener said she tried to find time to ride in between enjoying Christmas with her three children, aged 14, 17 and 19.
"On Christmas Day, I try to open all the stockings with the kids and then go out and ride to my parents, because they're doing lunch for us this year."
She said her family thought she was "completely nuts", but had been "very supportive".
"The first year I tried to do Christmas lunch and it was a disaster. I had done everything and said to my husband 'this is when you put the potatoes on'.
"When I came back, I found he couldn't cope with all the timings. I think we had lunch at 5pm. It was disaster, as we were supposed to be eating at 1pm."
"The following year mum agreed to cook."
She aims to ride 60km each day, with fellow members of cycling club Kent Velo Girls offering assistance.
"A lot of it is about mental strength," she said.
"This year will be difficult too as my father-in-law has just died. Christmas isn't always jolly and cycling does help clear my head."