It's a kick-start for the day, a jolt many of us need to power through, and one of the last socially acceptable addictions. It's only when we have to go without coffee that we realise how badly we rely on it.

The jitteriness and headaches from caffeine withdrawal can be so overpowering that many of us turn to decaffeinated instead, thinking surely it must be better for our health. But that might not be true, the experts say - not least because decaf coffee is not actually completely caffeine-free after all.

So if you can bear to make the switch to decaf, just how much better will it be for you?

Decaffeinated coffee is coffee that has been treated before roasting to strip the caffeine out of it, with "chemicals like carbon dioxide or by soaking the beans in water", says Dr Carlo La Vecchia, a professor at the University of Milan and an expert from the Institute for Scientific Information on Coffee.

This water-based method is sometimes referred to as the Swiss method and is thought to preserve the flavour of the beans more fully.

The names of other chemicals used in the decaffeination process - such as methylene chloride, a chemical banned from use in paint thinner, for its toxicity - might sound alarming. But the amount left in the beans after this process is negligible, Dr La Vecchia says: "None of them should remain in appreciable amounts."

This process does not completely strip the beans of their caffeine content, however, leaving behind "around 2 per cent, and in any case less than 3 per cent" of their natural caffeine levels. A normal cup of filter coffee contains around 100mg of caffeine, though the exact amount used will vary by brand, while a decaf filter coffee has closer to 2mg.

Normal caffeinated coffee has reams of health benefits, and fortunately most are entirely preserved in the decaffeination process.

One cup of brewed decaf coffee provides 2.4 per cent of the recommended daily intake of magnesium and 4.8 per cent of that of potassium, as well as 2.5 per cent of the niacin - a B vitamin - that you need each day.

Other benefits include:

"Coffee has so many benefits, mainly because it's made up of more than a hundred different plant chemicals that we know can impact health in a positive way," says Prof Sarah Berry, the chief scientist for nutrition company Zoe and a lecturer in nutritional sciences at King's College London.

It's the polyphenols in coffee - namely melanoidins and chlorogenic acid - that are to thank for most of its health-boosting benefits, including the effect on our diabetes risk, lowering it by up to 10 per cent in some cases.

Chlorogenic acid has also been linked to improved cardiovascular health, while melanoidins have a prebiotic effect, helping to diversify our gut bacteria.

But on top of all the individual benefits brought by the above, a regular coffee habit is linked to "a reduction in all-cause mortality", Prof Berry says, meaning that it could even make you live longer.

One 2017 study from Imperial College London found that people who drink three cups of coffee a day live longer than non-coffee drinkers. The study looked at the habits of half a million people across 10 European countries, including the United Kingdom.

While all causes of death were less likely to hit coffee drinkers, they had especially good protection against death from cardiovascular or digestive disease.

Being a lifelong coffee drinker could also slow cognitive decline as you age, according to one study of around 200 Australians over 10 years.

"We also know from a number of studies that drinking either caffeinated or decaffeinated coffee regularly is associated with a reduction in cardiovascular disease risk."

That said, "much less is known about the health effects of decaffeinated coffee," says Dr La Vecchia. "There is much more research into caffeinated coffee than decaffeinated coffee, because that's what people drink the most of."

"Apart from the caffeine, the chemical make-up of decaffeinated coffee is very similar to caffeinated coffee and it is still packed with those beneficial polyphenols," says Prof Berry.

The major difference - and downside - to decaf clearly is that, unlike with caffeinated coffee, it doesn't come with a temporary boost to alertness and focus - the reason that so many of us can't get through the morning without a cup of it.

As decaf coffee contains only a tiny amount of caffeine, this benefit is lost. The small amount of caffeine left over in a single cup of decaf coffee cannot be noticed by most people, though those who are sensitive to it might feel some effects (and drinking vats of it could still cause the jitters).

Research into the brain-health impacts of drinking decaf coffee regularly, versus caffeinated, is also more divided. However, some studies in animals have suggested that the same mechanisms that lower the risk of diabetes could also prevent cognitive decline, by helping the brain (and the rest of the body) to process glucose more effectively.

So, minus that temporary mental boost, drinking decaf coffee should still give you all the health benefits of drinking caffeinated coffee.

The amount of decaf coffee varies per brand. Here is the breakdown of the most popular kinds:

The other factor to consider when it comes to our health is how caffeine affects us individually. Every person "responds to caffeine very differently", says Prof Berry. If you're someone who is especially sensitive to caffeine - meaning you're prone to heart palpitations, nausea or an extreme "buzzy" feeling after just a cup or two - then switching to decaf coffee might make you feel better day-to-day.

Switching to decaf as the evening draws in could also help to improve your sleep, if you're a habitual coffee drinker. "People do become tolerant to coffee over time, but caffeine can still disrupt your sleep if you drink it in the evening," Prof Berry says.

"As a rule of thumb, I'd suggest that everyone stops drinking caffeinated coffee at 5pm and either switches to decaf coffee or something else that's caffeine free."

Decaf coffee is safe to drink, but the immediate withdrawal symptoms that come with cutting out caffeine - such as headaches, nausea and a lack of focus - can be unpleasant.

Due to the trace amounts of caffeine that decaf coffee still contains, however, it might still be unwise to get through vats of it in a day.

When it comes to drinking caffeinated coffee, "up to five a day is safe," Prof Berry says, but "you'll see the greatest benefits and the least side effects at two to three a day", and the same goes for decaf. "After five, I'd be slightly cautious."

"I would say that yes, decaf coffee is good for you, and caffeinated coffee is good for you too," says Prof Berry. "The one you drink should be a personal choice.

"There are huge benefits to drinking either, but with caffeinated coffee you'll get a boost to your cognitive functions, while decaf coffee promotes better sleep and might better suit people who are sensitive to caffeine."