And experts have now shared possibly the easiest weight loss trick ever – as it all happens while you sleep.
GP Dr Helen Lawal appeared on Channel 4's programme How To Lose Weight Well and asked expert Professor Michael Simmons an easy way to burn fat without doing exercises.
During the programme, Prof Simmons explained that the key is to "activate the brown fat" during sleep.
He said: "A good way to think of brown fat is the fat that keeps you thin.
"One gram of brown fat can produce 300 times more heat per gram than any other tissue in the body."
On average, we burn roughly 300 calories while sleeping but when the brown fat is activated as the body is at a cooler temperature, it helps burn extra calories.
And the way to do it is to bring down the room temperature to around 18C.
"Research has shown a temperature of between 18C and 19C may be sufficient," Prof Simmons added.
"Unless you feel cold you’re not going to switch on your brown fat. There’s no gain without pain."
The pair used thermal imaging cameras to compare the difference before and after the brown fat is activated.
Dr Lawal spent a night sleeping in a breezy room and the next day, pictures revealed a large white area appeared on her chest at around 4.30am. This meant the brown fat had been "switched on".
The result showed that she has burnt an extra 75 calories during the 8-hour sleep.
She asked how many extra calories could be burnt over a week and Prof Simmons replied: "It's going to be around 500 calories.
"That's potentially like having one meal less."
Dr Lawal added: "Or like a massive slice of cake. That's some decent amount for just laying cold when sleep isn't it?"