the neurobiological background for unihemispheric sleep is still unclear. in experiments on cats in which the connection between the left and the right halves of the brain stem has been severed, the brain hemispheres show periods of a desynchronized eeg, during which the two hemispheres can sleep independently of each other. in these cats, the state where one hemisphere slept nrem and the other was awake, as well as one hemisphere sleeping nrem with the other state sleeping rem were observed. the cats were never seen to sleep rem sleep with one hemisphere while the other hemisphere was awake. this is in accordance with the fact that rem sleep, as far as is currently known, does not occur unihemispherically.