Think of it how the heart adapts to effort. A person who is well rested can have his heart change quickly to the environment or effort. The person is more alert

Think of it how the heart adapts to effort. A person who is well rested can have his heart change quickly to the environment or effort. The person is more alert, can pump blood to any desired organ etc Meanwhile a person who is tired or sick or under stress will have more trouble adapting to the environment. Whatever happens, the heart beats at the same exact rhythm - but it shouldn't. It should go fast when required and go slow otherwise