Lord Brahmā, a human being and an ant all live for one hundred years, but their lifetimes of one hundred years are different from one another. This world is a relative world, and its relative moments of time are different. Thus the one hundred years of Brahmā are not the same as the one hundred years of a human being. From Bhagavad-gītā we understand that Brahmā’s daytime of twelve hours equals 4,300,000 times 1,000 years ( sahasra-yuga-paryantam ahar yad brahmaṇo viduḥ ). Thus the varṣa-śatam, or one hundred years, are relatively different according to time, person and circumstances. As far as human beings are concerned, the calculation given here is right for the general public. Although one has a maximum of one hundred years of life, by sleeping one loses fifty years. Eating, sleeping, sex life and fear are the four bodily necessities, but to utilize the full duration of life a person desiring to advance in spiritual consciousness must reduce these activities. That will give him an opportunity to fully use his lifetime.