Children's, Intermediate and Advanced Online English Dictionary & Thesaurus

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REM sleep a recurrent phase of sleep, characterized by rapid side-to-side eye movement, dreaming, and inhibition of muscle movement.
sleep mode a power-saving mode of operation to which a computer or other machine reverts when idle, in which power to the display and to other nonessential parts of operation is deactivated.
sleep tight to sleep well or soundly.