Can you imagine crashing waves freezing instantly into arctic icecaps-within your sleeping brain? Well, maybe not, but from the question you might be able to infer that the sleeping brain is far from tranquil. It's neural activity switches back and forth between states of crashing waves and frozen icecaps. This analogy may be confusing, so allow me walk you through the brain's ocean waves first.

When researchers first discovered how to measure electrical signals on the skull, using electro encephalogram recording (EEG), people immediately noticed that the sleeping brain is not resting at all (in the sense of brain activity). When a human or other mammal falls into sleep, electrodes recording from the skulls surface will start to show what is termed 'slow-wave' activity. Large in amplitude, low in frequency (hence the name "slow-wave"), this type of activity can be seen over a large part of brain skull. As one falls deeper into sleep, this slow-wave activity appears more frequently, up to one cycle per second (1 Hz). We call this part of sleep "slow-wave sleep." Because slow wave can be characterized by highly correlated neural activity, it is also referred to as the "synchronized state."

However, around 90 minutes into the sleep cycle, dramatic events occur-the brain freezes, at least in terms of large-scale oscillations-and the big, slow waves over the entire brain are nowhere to be found. Instead, high-frequency, small-amplitude activity prevails, with greatly reduced correlation. This phase is appropriately referred to as the "desynchronized state." In this phase, our eyes move rapidly back and forth in what scientists call "rapid eye movement" (REM) sleep. Most of our dreams happen during REM sleep and, in adults, this activity occupies about 20-25% of the total duration of sleep (or about 90-120 minutes per night). However, REM sleep occurs over approximately four or five cycles throughout the night, with each cycle being slightly longer than the previous one. Slow wave oscillation and memory consolidation

Why do we care whether the brain is in a slow wave or REM state? One reason is that controlling sleep stages is critical for narcolepsy, a strange sleep disorder that affects about one out of every 1,000 people in the United States. Why is it important for the other 999 people as well? The answer to this question is actually one of the most active research areas in neuroscience: Slow wave oscillation appears to contribute to memory consolidation. That is, without slow wave sleep, memories don't get "burned" into your brain.