REM sleep was a stage of sleep necessary to most beings. If a person did not get the acquired amount, there could be repercussions, ranging from depression, exhaustion and hallucinations all the way to a possible psychotic break. Droids could, in a manner of speaking, also sleep when they entered a powered-down state from time to time. Reportedly, Darth Sidious killed his Master Darth Plagueis while the latter was sleeping.