Magnus Bowden, group gym development manager for Cannons, replies: The increase in heart rate in these two examples is unlikely to be high enough or for long enough to have any real benefits. When exercising you can control by how much and for how long the heart rate stays elevated; this is not true with coffee or horror films. Stress hormones are produced when drinking coffee or watching horror movies as the body reacts in a 'fight or flight' response. In the case of exercise, these hormones will be used up, but they can damage the body over time if not used, possibly leading to headaches, IBS, chronic fatigue and depression.