Heating or cooling the body creates a small stress, she says, which may trigger adaptive or protective responses.

The science behind the sauna

That idea underpins the appeal of saunas, which nowadays at gyms and spas are rarely empty.

For some people it's a reward after exercise while for others, it's the main attraction and many regulars swear by it, convinced that 15 minutes of intense heat does wonders for their body and mind.

And there's no doubt it can feel good.

"When you sit in a sauna and work up a sweat, you may feel looser, freer, a bit more mobile and your aches and pains may resolve themselves a bit," Dr Massey tells BBC's What's Up Docs podcast.

"So there's definitely some benefit to using the sauna but the question is whether that's a long-term health benefit or more of a psychological one."

Dr Massey explains that a recent study put people repeatedly in hot tubs and the results showed changes in insulin and blood pressure.

"We're starting to explore whether heating the body could help people living with lifelong conditions."

Still, she urges caution when it comes to bold health claims as robust scientific evidence is still limited.

"We've never really done a proper sauna trial," she says. "I suspect we'll find benefits in the future, but we're not there yet."

For now, she says, it's reasonable to enjoy the ritual for how it makes you feel, without assuming it's a guaranteed shortcut to better health.

And if you do try saunas or hot tubs, Dr Massey advises caution, "ease yourself in gradually" and check with your GP first if you have any underlying health conditions or are pregnant.

What about cold water swimming?

Some people like to push things in the opposite direction.

Cold-water swimming groups are growing in popularity, with early-morning dips now a familiar sight at beaches, lakes and rivers.

Dr Massey, who has swum the channel and competed in the world ice-swimming championships, goes cold-water swimming once a week but only spends a couple of minutes in the water.

She finds it "painful" at first, but that initial shock is exactly what people are chasing.

"When you first immerse, you get an involuntary gasp and rapid breathing," she explains. Heart rate and blood pressure jump and stress hormones such as cortisol and adrenaline surge.

"That response peaks at about 30 seconds and then drops away very quickly," she adds.

Repeated exposure reduces the shock response and after several short swims, it can be cut by around 50%.

As with saunas, the big question is whether the benefits come from the temperature itself, or everything wrapped around it.

Cold-water swimming is usually done outdoors and with others. "It's an activity with multiple mechanisms - nature, movement and social connection and they are all interlinked," Dr Massey says.

Podcast host Dr Chris van Tulleken agrees it's difficult to separate those factors: "We don't know if the benefit is the cold water itself or being with a group of fun people and doing something challenging together."

Find something you enjoy

For Dr Massey, though, the starting point isn't temperature at all, but enjoyment.

"I speak to people who do Parkrun every week, and describe exactly the same kind of buzz as other people get from cold water swimming," she says.

"What matters is finding something you enjoy, can do regularly, and ideally can share with others, as building a friendship group around an activity can reduce psychological stress."

That could be anything from gardening, birdwatching, joining a choir or simply walking with friends.

So even if saunas and cold swims are not essential for good health, says Dr Massey, they may still offer something positive.