The key to the survival and reproduction of all mammals is the ability to acquire energy. If mammals cannot acquire energy to carry out their physiological processes, then lack of energy access can be viewed as a selective pressure. The inability to acquire food in a particular environment selects for sleep expression as a method of energy conservation.
Studies performed by Nelson and others reveal that energy constraints play an essential part in mammalian sleep patterns. In their experiment, researchers monitored the sleep cycle of three bears over the course of three years. During the winter seasons when very minimal amounts of nutrients was available, they found that bears entered a deep state of sleep known as hibernation where very minimal amounts of energy was expended to keep the bear alive. According to the study, at the end of the ninety-seven day winter sleep, the bears had not urinated or defecated, and had lost 25% of their body mass. However, at the end of winter sleep the bears were almost in perfect water balance with normal concentrations of plasma and red blood cells (Nelson et al.1973).
Additional studies performed in 2008 by Capellini and others further support this hypothesis. In the study carried out, researchers extracted data from various scientific articles regarding the sleep traits of fifty-six terrestrial mammals. The information gathered in their analysis includes; sleep period, sleep distribution, body mass, predation risk, and length of REM and NREM sleep states. By comparing these components, the researchers were able to determine whether predation risk or energetic conservation had the greatest impact in driving mammalian sleep patterns. According to the results, there is an association between larger body mass and increased sleep periods.