Not just any old bath will do the trick. While Charles emphasises that finding the 'ideal dose' of passive heating to reap the magical health rewards is "likely to be very complex and highly reliant on the type, duration, and temperature", research so far suggests that you'd want to be sitting up to your shoulders in water of around 40C (104F) for an hour to gain some of the benefits. That means you're better off in a hot tub, where the temperature can be controlled, than a bath, which will cool relatively quickly. "In the real world, the key is to find the correct balance between a heating stimulus that is tolerable/enjoyable and capable of improving health," he adds. After all, to see the benefits of any health strategy, you need to able to stick to it in the long term. And sweating your face off in a boiling hot bath probably might in fact be less appealing than going for a jog, or cutting down on biscuits.