It is suggested that environmental variables, and gravity in particular, are the main determinants of sleep duration. Assuming that the rapid eye movement (REM) sleep state depends on the influence of gravity allows a better understanding of sleep across the animal world. This paper is based on numerous results already published on sleep behaviors, sleep postures, sleep durations, and the weights of many species. The difference between the sleep behavior of aquatic animals and terrestrial mammals is explained by the effect of gravity in dif- ferent environments: ocean or land. Archimedes' principle applied to aquatic animals shows that their weight is minimal and this may explain the lack of REM sleep. The fact that cats or rats in unsafe positions above water do not experience REM sleep is explained by the fear of atonia, a complete relaxation of the antigravity muscles, which happens during REM sleep, completing the process of reduced sensitivity to the environment: light, sound, contact, etc. Furthermore, a very clear dependence of sleep duration on weight is obtained when plotting the data for 76 terrestrial mammalian species, showing that sleep and gravity are related. Another dependence of sleep duration, on gestation time, is presented, and this shows the difficulty of interpreting sleep and its relation to a single parameter.