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During sleep, our bodies are relatively inactive and we have limited awareness of the outside world, but our brains remain active and we pass through different degrees of consciousness in recurrent cycles. On falling asleep, we sink rapidly into successively deeper stages of sleep, but after about 90 minutes the process reverses, to a point where brain activity resembles the waking state. This stage is characterised by rapid eye movement [RME], but although the brain is very active, the muscles of the body are paralysed and waking someone is harder than at any other stage. We typically go through three of four cycles of alternating REM and non-REM sleep each night, and each time the non-REM periods become shorter and less deep, while the periods of REM sleep lengthen. It is during REM sleep that we dream. Deprivation of REM sleep has been shown to cause REM rebound — longer and more frequent REM sleep later in the cycle — suggesting that one purpose of sleep is to dream, perhaps to sort out our thoughts and prepare our brains for new input.