All around the world, people fall asleep to different sounds. Maybe you hear a snoring dog, whooshing waves, noisy traffic, chirping crickets, a soft lullaby, or raindrops.

These kinds of sounds can actually help different parts of our body kick into gear, said my friend Devon Hansen. She’s a sleep researcher at Washington State University.

Her lab often investigates two of our body’s systems by hooking people up to a heart monitor. Monitoring a person’s heartbeat can help her see which of these two systems are active.

Illustrated, cartoon cat with labcoatOne system is the sympathetic nervous system. If you ever feel threatened, this system helps you decide whether to run away or confront the situation.

Another system is the parasympathetic nervous system. This one helps you stay calm, rested, and also digest your food–without even having to think about it. When you are in “fight or flight” your heartbeat speeds up. When the parasympathetic nervous system is active, your heartbeat is slower.

A fire alarm going off, a lion roaring nearby, or someone screaming might startle you and kick in that “fight or flight” system. You’re suddenly alert and awake to respond to danger. But the sounds of rain or ocean waves aren’t usually very threatening noises. They activate the parasympathetic nervous system and your body relaxes.

These kinds of soothing sounds can also muffle noises that might keep you awake or distract you, such as a howling dog or loud traffic.

Hansen said it’s important to remember that sound itself doesn’t make you fall asleep. It’s really that your parasympathetic nervous system is activated, which is what relaxes the body. Once the body is relaxed, it will naturally fall asleep.

You can try a sleep experiment of your own at home. Find a metal spoon or fork. Write down the time you go to bed. Then grab a metal tray and put it on the ground next to your bed. Once you are ready for bed, hold the utensil in your hand over the tray.

When your body and hand relax, you will drop the fork or spoon. The clatter will probably wake you up. Then write down the time you woke up. Subtract the time difference and you’ll see how long it takes you to fall asleep. You can try this while listening to different sounds, like the sounds of a forest, rain, or the ocean.

Hansen adds that putting on soothing background sounds can be helpful when falling asleep, but it’s not as helpful for staying asleep. If your speakers are on all night, they could be waking you up a lot without you realizing it. But it could explain why you might be tired the next morning. A good night’s sleep is important for humans, especially if you want to stay sharp and keep thinking up great science questions.

Dr. Universe