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Most of the memory consolidation occurs in the first two hours of slow-wave, NREM sleep, and during the last 90 minutes of REM sleep in the morning. This is why drinking and drugs (including sleep medications) can interfere with memory, because that crucial first sleep cycle is compromised by intoxication. And this is why sleep deprivation leads to memory loss--because the crucial 90 minutes of sleep at the end is either interrupted or never occurs. And you can't make up for lost sleep time. Sleep deprivation after a day of learning prevents sleep-related improvement, even three days later following two nights of good sleep