As we fall into sleep, our brain stays active and fires into its editing process—deciding which memories to keep and which ones to toss.
The initial transformation happens quickly. The human body does not like to stall between states, lingering in doorways. We prefer to be in one realm or another, awake or asleep. So we turn off the lights and lie in bed and shut our eyes.
If our circadian rhythm is pegged to the flow of daylight and dark, and if the pineal gland at the base of our brain is pumping melatonin, signaling it’s nighttime, and if an array of other systems align, our neurons swiftly fall into step.
Neurons, some 86 billion of them, are the cells that form the World Wide Web of the brain, communicating with each other via electrical and chemical signals. When we’re fully awake, neurons form a jostling crowd, a cellular lightning storm.
When they fire evenly and rhythmically, expressed on an electroencephalogram, or EEG, by neat rippled lines, it indicates that the brain has turned inward, away from the chaos of waking life. At the same time, our sensory receptors are muffled, and soon we’re asleep.
Scientists call this stage 1, the shallow end of sleep. It lasts maybe five minutes. Then, ascending from deep in the brain, comes a series of electric sparks that zap our cerebral cortex, the pleated gray matter covering the outer layer of the brain, home of language and consciousness. These half-second bursts, called spindles, indicate that we’ve entered stage 2.
Our brains aren’t less active when we sleep, as was long thought, just differently active. Spindles, it’s theorized, stimulate the cortex in such a way as to preserve recently acquired information—and perhaps also to link it to established knowledge in long-term memory.
In sleep labs, when people have been introduced to certain new tasks, mental or physical, their spindle frequency increases that night. The more spindles they have, it seems, the better they perform the task the next day.
The strength of one’s nightly spindles, some experts have suggested, might even be a predictor of general intelligence. Sleep literally makes connections you might never have consciously formed, an idea we’ve all intuitively realized. No one says, “I’m going to eat on a problem.” We always sleep on it.
The waking brain is optimized for collecting external stimuli, the sleeping brain for consolidating the information that’s been collected. At night, that is, we switch from recording to editing, a change that can be measured on the molecular scale. We’re not just rotely filing our thoughts—the sleeping brain actively curates which memories to keep and which to toss.
It doesn’t necessarily choose wisely. Sleep reinforces our memory so powerfully—not just in stage 2, where we spend about half our sleeping time, but throughout the looping voyage of the night—that it might be best, for example, if exhausted soldiers returning from harrowing missions did not go directly to bed.
To forestall post-traumatic stress disorder, the soldiers should remain awake for six to eight hours, according to neuroscientist Gina Poe at the University of California, Los Angeles. Research by her and others suggests that sleeping soon after a major event, before some of the ordeal is mentally resolved, is more likely to turn the experience into long-term memories.
Stage 2 can last up to 50 minutes during the night’s first 90-minute sleep cycle. (It typically occupies a smaller portion of subsequent cycles.) Spindles can arrive every few seconds for a while, but when these eruptions taper off, our heart rate slows. Our core temperature drops. Any remaining awareness of the external environment disappears.
We commence the long dive into stages 3 and 4, the deep parts of sleep.