many herbivores, like ruminantia (such as cattle), spend much of their wake time in a state of drowsiness, which perhaps could partly explain their relatively low need for sleep. in herbivores, an inverse correlation is apparent between body mass and sleep length; big mammals sleep less than smaller ones. this correlation is thought to explain about 25% of the difference in sleep amount between different mammals. also, the length of a particular sleep cycle is associated with the size of the animal; on average, bigger animals will have sleep cycles of longer durations than smaller animals. sleep amount is also coupled to factors like basal metabolism, brain mass, and relative brain mass. the duration of sleep among species is also directly related to basal metabolic rate (bmr). rats, which have a high bmr, sleep for up to 14 hours a day, whereas elephants and giraffes, which have lower bmrs, sleep only 3–4 hours per day.