ized countries
undoubtedly contribute very largely to neurasthenia and allied
diseases. Improvements in artificial lights have contributed largely
toward the increase of the evil of late hours, injurious not only
through the loss of sleep entailed, but also because of the eye-strain
incidental to strong artificial lights and the drain on the nervous
system. If civilized man would follow the example of primitive man and
of many of the birds and animals in retiring to bed with the coming of
darkness and arising with the appearance of daylight, this one change
would revolutionize the health of the whole human race.

How much sleep do we need? This is a question that cannot be answered
arbitrarily as applying in all cases. Individuals differ. Without
doubt, some require more sleep than others.

Thomas A. Edison, who is an extraordinary man, not only in respect to
his vitality but in every other characteristic as well, has
frequently been quoted as saying that most men and women sleep too much.
Mr. Edison himself claims to maintain the best of health with from three
to five hours' sleep out of every twenty-four. We have heard of other
cases too, of men and women with exceptional vitality, who have seemed
to thrive on four or five hours' sleep. It is possible that this small
allowance of sleep may be sufficient in such cases, but if so, it is
undoubtedly due to the exceptionally powerful organism which these
particular persons have inherited.

No definite rule can be laid down as to the amount of sleep required by
different individuals, for those possessing the greatest amount of
vitality and the strongest organisms will require less sleep than those
of limited vitality and weak functional powers. Those possessing a
strong functional system and great vitality are able to build up energy
during sleep and recuperate from the exertions of the preceding day more
rapidly than can those less favored in this respect. In other words, a
very strong man can be quickly rested. His system can more rapidly
than that of a weak man repair the wear and tear of his daily work. The
man or woman with limited strength and a less vigorous functional
system would require a longer time in which to recuperate. Therefore,
what would hold good in the case of such an extraordinary man as
Mr. Edison cannot be depended upon in the case of the average man or
woman, and certainly will not meet the needs of those who