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You feel it the moment your alarm goes off—that soggy, sickly sensation of having slept all wrong. You pour OJ on your cereal and get 10 steps out of the house before realizing your contacts aren’t in. "Today’s going to be so much fun!" you think.

Cognitive scientists still can’t explain just why you need sleep. But there’s plenty of research showing how a lack of it cripples your mind, says Namni Goel, Ph.D., a biological psychologist and sleep expert at the University of Pennsylvania. 

Books have been written about insomnia and its effects. But Goel and other sleep researchers provide a basic map of the sleep-deprived brain.

1 Night of Poor Sleep or Less Than 6 Hours
Cognitive throughput—the speed with which your noodle processes information—slows down after a single night of tossing and turning, Goel says. [Tweet this fact!] Too little sleep disrupts the neural pathways that allow information to travel smoothly from one area of your brain to another. Your thoughts are like a train diverted onto the wrong set of tracks and taking the slow route to town. As a result, your working memory, which handles decision-making and problem solving, struggles to perform normally when you’re wiped out, Goel says. That’s why you may have a harder time choosing what to wear and your normal job tasks take you twice as long when you’re tired.

Research from the University of California, San Diego also linked a single night of poor sleep to less-efficient filtering, meaning your brain has issues picking out important or relevant nuggets of information from all the insignificant junk your senses absorb. There’s also some evidence linking a night of bad sleep to spikes in brain chemicals like serotonin during the day. Serotonin is elevated among those with depression, which may explain why you feel emotionally down when you’re fatigued.

RELATED: 12 Common Sleep Myths Busted!

Several Nights of Poor Sleep or 1 Night of No Sleep
All of the brain functions mentioned above suffer more severely as you clock fewer hours of rest, Goel says. You also experience serious hormonal disruptions. Your body’s ghrelin levels—which stimulate feelings of hunger—jump nearly 15 percent, according to a study from Stanford University. As a result, some researchers have recorded a 2.2-pound weight jump in a single week among people who were sleeping poorly, Goel says. The same Stanford study found that several nights of bad sleep lowers your body’s levels of leptin, a hormone that regulates energy. That leptin drop-off helps explain why you feel so sluggish when you’re sleepy, Goel adds.

Your brain also struggles to curate and sort new memories, shows research published in the journal Neuroscience. You’re able to absorb new information, Goel explains, but your brain has problems accessing that info and putting it to good use. On top of that, disruptions in the prefrontal cortex region of your noggin can leave you feeling continually distracted, while your reaction time plummets, Goel adds. This makes tasks like driving difficult—and dangerous.

Weeks of Poor Sleep or Several Nights Without Rest
When you sleep, neurons and cells in your brain also rest. But among insomniacs—loosely defined as someone who sleeps four hours or less for five nights or more—those cells and neurons never get an adequate snooze. As a result, some die off, while others become clogged with proteins that sleep would normally have cleared away, shows a study from the University of Pennsylvania. This can lead to long-term, irreversible brain damage and problems related to attention or information processing, the Penn study authors say.

The brains of insomniacs also show increased activity in the motor cortex and several other brain regions, indicates a study from Johns Hopkins University. That may seem like a good thing, but it’s actually a sign that you’re brain isn’t working efficiently, the researchers say. Like a dog that barks at every noise it hears, your mind is overreacting to its workload, and will wear itself out. As that happens, you become worse and worse at handling simple tasks and making smart decisions, the study suggests.

There’s also evidence too-little sleep can actually spur genetic changes in your brain—and not the kind you want. A team from the University of Washington studied more than 1,700 twins, and found those who regularly slept less than five hours a night were at twice the risk for depression. The study authors say a lack of sleep activates certain genes related to the blues.

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