Cows may not be the lazy creatures we’ve been led to believe, but they do need daily periods of deep or ‘REM’ sleep to maintain their immune function and energy levels. In fact, they can lose a quarter of their milk production if they don’t get enough rest.
While the majority of a cow’s sleep takes place at night, they do nap throughout the day in varying degrees of depth and length depending on their needs. Cows will take these brief naps to conserve their energy for activities such as grazing or milking.
The amount of sleep a cow gets will depend on how much energy she has to spare that day and the time of year. In general, adult dairy cows need around 4 hours of sleep a day of which less than an hour is REM sleep. Cows will also spend a considerable amount of time in NREM (non-rapid eye movement) sleep and in a drowse state that is somewhere between sleeping and awake.
Researchers have found that cows can change between these three states of wakefulness by changing their lying posture. Sleeping with the head elevated or lowered can indicate NREM sleep, while lying in a prone position (with the head resting on the flank) indicates REM sleep. A drowse position could be indicative of both NREM and REM sleep, but it is unlikely that cows would be able to enter REM sleep in a standing or lying down position.