Have you ever been sleeping peacefully, enjoying a dream so good that you wished you could keep sleeping longer? Or have you ever had a dream that was so weird or scary that you were relieved when you woke up?

Dreams happen all the time. Sometimes they're delightful and other times they can just be plain weird! They're a mystery that no one can really explain.

Some dreams — like the one about the skunk who surfs up the side of Mt. Everest, only to hang glide back down into the jungles of the Amazon while knitting a scarf — are just crazy! Other dreams — called nightmares — may be frightening because they contain monsters or dangerous situations.

Nightmares happen to everyone, even adults, from time to time. They can make you feel anxious, scared or just unsettled. Don't worry, though. They're not real and they can't hurt you.

Nightmares — and other types of dreams — occur during a period of sleep that occurs several times each night. It's called REM sleep, which stands for Rapid Eye Movement sleep. When you sleep, your brain never turns off. About every 90 minutes or so, your brain cycles through a REM sleep cycle, in which your eyes move rapidly back and forth behind your closed eyelids.

During REM sleep, you often have dreams, some of which may be crazy or scary. Crazy or scary dreams may be the body's way of dealing with stressful things you experienced during the day. Nightmares might also sometimes be caused by watching scary movies or reading scary books right before bed, as you take those recent images and memories into sleep with you.

Not all crazy dreams are purely mental phenomena, though, and they don't always occur when you're asleep. Sometimes crazy daydreams are hallucinations caused by physical changes in your body. For example, if you are sick, a high fever can sometimes cause hallucinations, which are just extremely vivid — and often crazy — dreams that happen when you're awake and see things that aren't really happening.

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