Our ears and brains continue to process sounds even when we’re deep asleep. That’s how a mother can awake to her child’s cries, for example. But she wouldn’t wake up because of the noise of chirping crickets or a whirring fan.

Even in our deepest state of sleep, some parts of the brain remain active. This is a vigilance function, a survival instinct that lets the body decide whether or not to wake up when you hear sounds. 

Science Daily reported a study by a Johns Hopkins undergraduate student that pinpointed areas of the brain’s frontal lobe that respond to sounds even when sleeping. Even though sounds may not wake you up from sleep, they can change body functions such as increasing heart rate and releasing hormones.

Even though scientists don’t know exactly how we monitor our environments when we sleep, they know that it can influence the stages of sleep.

How Sounds During Sleep Affect The Stages of Sleep

Every night, a normal adult goes through four to six sleep cycles. Each of these cycles consists of four stages each with different characteristics.

  • Stage 1 (light sleep): You’re just starting to sleep but aren’t completely relaxed. You’re still paying attention to your environment, your body may twitch, and it’s very easy to wake up. This is what happens during a short nap.
  • Stage 2: Heart rate, breathing, and body temperature start to drop. The brain also slows down, so it’s harder to wake up.
  • Stage 3 (deep sleep): This is a slow-wave sleep in which your brain disconnects from the rest of the world. Your breathing, heart rate, and temperature drop significantly. You retain a slight awareness of the outside world, but it’s hard to shake you from this sleep.
  • Stage 4 or Rapid Eye Movement (REM) Sleep: This is the stage when you dream. All brain activity is temporarily paused, and it is the deepest stage of sleep during which you can’t be awakened by sounds. 

Stages 3 and 4 are vital for good health and brain function. Stage 3 sleep is when the brain consolidates memories, relaxes muscles, and starts self-repair. Stage 4 or REM sleep is critical for immune and cognitive functions, learning, and creativity, and is responsible for vivid dreams.

However, sounds during sleep stop you from reaching the two-deep stages of sleep. It causes the body to react to all sounds, become excited, and release hormones such as adrenaline and cortisol.

Over time, chronically disturbed sleep and insomnia can lead to health problems such as obesity, diabetes, and hypertension (high blood pressure.)

How White Noise Affects Sleep

Studies have shown that we can hear even when we’re asleep, except in REM sleep. However, not all sounds will arouse your body and increase vigilance. Sounds that the brain determines as soothing and non-threatening—called white noise—can actually improve sleep quality.

White noise, such as a humming fan, is continuous, low-frequency sound without any high-frequency peaks. It helps to block out sharp outside noise, a bit like how a pair of noise-blocking earphones or headphones work.

With less interruption in your sleep cycles, you will sleep better and reach all four stages of sleep. White noise also helps people with hearing loss or tinnitus who, ironically, find it hard to sleep properly. While noise helps them sleep by relaxing their brains more and introducing background sounds.

Your TV or radio is a terrible choice for background noise because it has frequency peaks and unexpected sounds. If you’re having trouble sleeping due to ambient noise, consider getting a whilte noise machine.