As you sink into sleep, you enter a state called hypnagogia, similar to hypnosis. You may hear strange noises or snatches of imaginary conversations, experience odd physical sensations, and see vivid hallucinations of geometric shapes. Your muscles may twitch involuntarily. Nobody is sure why. Nor does anyone know why, as we enter deeper sleep, our white blood cell count rises and eventually we begin to dream. But after decades of careful observation, scientists have figured out that there are a few things we do better when we're sleeping than when we're awake.

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Forming accurate memories

Most neuroscientists agree that sleep is when we organize memories for long-term storage. People whose sleep is disturbed after studying have far more imperfect recall than people who get a good eight hours of shuteye. But a study published a couple of weeks ago shows that you can use simple memory reinforcement techniques while you're sleeping that will make your recall better than average.

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A group of researchers gave a group of student volunteers a simple recollection test. They had to match pairs of cards, take a nap, and then try to remember all the cards they'd matched up. But some of the volunteers were given this memorization task in an environment with a slight odor. While those volunteers took a nap, the smell was pumped into the place where they slept. The idea was to reinforce their memories with a smell associated with the task they'd just done. And it worked. The people who slept with the smell performed significantly better than the control group after their nap. In fact, the researchers say, their recall was "nearly perfect."

This doesn't mean you should study while sniffing perfume, though you're welcome to try. But it does show that what our brains do while we sleep has a lot to do with how well we remember what we've done while we're awake.

Heal from infection

We don't just need sleep to improve our minds - we need it to heal our bodies. Physiologist Marc Opp argues that it's possible that sleep is part of our immune system, and that we may have evolved sleep alongside our other bodily defenses against infection. In fact, our ability to dream appears to be connected at a molecular level to our healing abilities. When researchers reduce the levels of proteins used in healing wounds and fighting infection, it also reduces REM sleep. Raising the levels of those same proteins causes people to dream more. So when you go to bed with a cold and wake up feeling better after a night of weird dreams - well, there's a good reason for that.

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Plus, our white blood cell count increases while we sleep. We can actually defeat parasites in our bodies while we snooze. Opp writes:

Analysis of sleep times of 12 mammalian species for which data also are available with respect to parasitic load indicates that a 10 hour increase in sleep time is associated with a 24-fold decrease in parasitism.

That's a pretty incredible decrease.

The tragic corollary to all of this is that lack of sleep makes you more prone to infection. Mice who were sleep deprived started developing sores on their paws and tails. Humans who are sleep-deprived are probably more susceptible to infection, too.

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Deal with stress

Dreaming also appears to be one of the main ways we maintain emotional equilibrium. Sleeping appears to organize our emotions in the same way it organizes and solidifies memories. Researchers report that sleep helps people recognize other people's emotional states, maintain calm in the face of difficult situations, and even develop feelings of trust more easily.

In a recent article from Sleep Medicine Reviews, two Belgian researchers echo many other neuroscientists in their field when they write that REM sleep - the dream state - seems crucial to restoring emotional well-being. We handle stress better after undergoing certain brain activities that only dreams can unleash. They write:

Sleep seems restorative in daily functioning, whereas deprivation of sleep makes us more sensitive to emotional and stressful stimuli and events in particular. The way sleep impacts next day mood/emotion is thought to be affected particularly via REM-sleep, where we observe a hyperlimbic and hypoactive dorsolateral prefrontal functioning in combination with a normal functioning of the medial prefrontal cortex, probably adaptive in coping with the continuous stream of emotional events we experience.

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You're not just handling stress better because you've rested. In fact, it's because dreams make your brain so active at night that you're able to tackle problems better after a good night's sleep.