Charles H. Spurgeon on Psalms 104:20

Thou makest darkness, etc. It would be interesting to consider the wonderful adaptation of the length of the day to the health of man, and to the rigour and perhaps existence of the animal and vegetable tribes. The rejoicing of life depends so much upon the grateful alternation of day and night. For a full consideration of this subject I must refer the reader to Dr. Whewell's Bridgewater Treatise. The subjoined extracts may, however, aid reflection. The terrestrial day, and consequently, the length of the cycle of light and darkness, being what it is, we find various parts of the constitution both of animals and vegetables, which have a periodical character in their functions, corresponding to the diurnal succession of external conditions; and we find that the length of the period, as it exists in their constitution, coincides with the length of the natural day. The alternation of processes which takes place in plants by day and by night is less obvious, and less obviously essential to their well being, than the annum series of changes. But there are abundance of facts which serve to show that such an alternation is part of the vegetable economy. . . . "Animals also have a period in their functions and habits; as in the habits of waking, sleeping, etc., and their well being appears to depend on the coincidence of this period with the length of the natural day. We see that in the day, as it now is, all animals find seasons for taking food and repose, which agree perfectly with their health and comfort. Some animals feed during the day, as nearly all the ruminating animals and land birds; others feed only in the twilight, as bats and owls, and are called crepuscular; while many beasts of prey, aquatic birds, and others, take their food during the night. These animals, which are nocturnal feeders, are diurnal sleepers, while those which are crepuscular sleep partly in the night and partly in the day; but in all, the complete period of these functions is twenty-four hours. Man in like manner, in all nations and ages, takes his principal rest once in twenty-four hours; and the regularity of this practice seems most suitable to his health, though the duration of time allotted to repose is extremely different in different cases. So far as we can judge, this period is of a length beneficial to the human frame, independently of the effect of external agents. In the voyages recently made into high northern latitudes, where the sun did not rise for three months, the crews of the ships were made to adhere, with the utmost punctuality, to the hallit of retiring to rest at nine, and rising a quarter before six; and they enjoyed, under circumstances apparently the most trying, a state of salubrity quite remarkable. This shows, that according to the common constitution of such men, the cycle of twenty-four hours is very commodious, though not imposed on them by external circumstances." —William Whewell (1795-1866).