Quiz: The Science of Sleep

REM, or rapid eye movement sleep, is the last of the five stages of sleep. During REM sleep, muscles in your arms and legs are temporarily paralyzed. When people wake up from REM they often say they’ve had vivid and bizarre dreams. Not being able to move may prevent you from acting out your dreams.

Most adults need around 8 hours of sleep. As you get older, you may still need 8 hours of sleep, but you might sleep lighter and struggle to get the hours you need in one block. Older adults wake up more often and stay awake longer. Good sleep habits, such as staying away from caffeine and alcohol and going to bed at night when you're tired, can help.

When humans sleep, so does their entire brain. When dolphins and whales sleep, a part of their brain stays awake. They need to stay conscious so they can surface out of the water to breathe.

Remember how we said you have five stages of sleep? Those stages together make up one cycle, which lasts around 90 minutes total.

You go through several sleep cycles over the course of a night. Getting out of bed can be hard if your alarm goes off in the middle of one, so try setting a wake-up time that’s a multiple of 90.

 If you go to bed at 10 p.m., you may feel more refreshed at 5:30 than you would if you got a full 8 hours of sleep because you’re getting up at the end of a cycle.

Sleepwalking can be more than walking around the room or house while asleep. Some people just sit up in bed and look around, while others leave their homes and drive long distances.If you find someone in your home sleepwalking, wake that person up to protect them from harm. People who’ve been depressed or had obsessive compulsive disorder are more likely to sleepwalk than people without those issues.

It might be tempting to nap the afternoon away, but you should limit your daytime shuteye to 30 to 60 minutes. If you sleep for longer, you might fall into deep sleep and have a hard time waking up. Many people who nap say they are more alert in the afternoon and evening. But napping too long can make you stay up later and sleep less at night.

Turning on a light when you get up could hurt your sleep. At night, your body produces a hormone called melatonin, which makes you drowsy. This usually starts around 9 p.m. Melatonin is produced only in places that are dimly lit. Bright indoor lighting – like the kind in bathrooms – can block the release of melatonin. It can also be affected by lesser light from TVs, computers, tablets, and smartphones.

Scientists don’t know much about how or why you dream, but they do know how much time you spend doing it. For most people, it’s more than 2 hours each night. Dreams almost always happen during REM sleep. Most mammals and birds show signs of REM sleep. Reptiles and other cold-blooded animals don’t.