In the above paragraphs we have tried to make clear that the blood is kept in motion through the body by the work of the heart, and that the heart’s activity varies in accordance with the needs of the body; in muscular exercise there is a great increase in metabolism, which means a greatly increased demand both for food supplies and for oxygen. To meet this increased demand it is necessary that the blood circulate more abundantly, and in the automatic speeding up of the heart through the nervous system we have the means by which this is done. In the case of strong emotion, as already emphasized, the bodily reactions are such as put the body into the best possible condition for meeting the emergency. Evidently a quickened heartbeat, by insuring abundant supplies of oxygen and of foodstuffs, contributes to this end. The slowing of the heartbeat, when one lies down, is evidently helpful in enabling the heart itself to recover from any strains that may have been put upon it. The heart is a muscle, and like any other muscle carries on its functional metabolism, which means that it is oxidizing fuel materials and producing waste products. Since it is absolutely necessary that the heart go on beating regularly year in and year out for perhaps eighty or a hundred years, any relief from activity that it can get by slowing down during sleep is evidently an advantage. It has been calculated that the heart muscle really enjoys an “eight-hour day,” by which is meant that on the average the functional metabolism of contraction is going on in heart muscle only about one-third of the time, eight hours out of each twenty-four. During the active waking time the metabolism takes a larger percentage than that, but during sleep enough less to even up.