It’s a common experience to be roused from sleep by a loud noise. You might wake up to the sound of an alarm clock, a slamming door, or a clap of thunder. This seems to suggest that we can hear while we’re sleeping. But what does the science say? Can our brains actually process sounds and noises while we’re unconscious?
The short answer is: yes, we can hear while asleep, but not in the same way as when awake. Let’s take a closer look at what’s going on in the brain during different sleep stages and how that impacts our ability to hear and respond to sounds.
The Stages of Sleep
To understand how hearing works during sleep, we first need to understand the different stages of sleep. There are two basic types of sleep: non-REM and REM.
Non-REM sleep progresses through three stages:
Stage 1
Stage 1 is the lightest stage of sleep. During this stage, you drift off to sleep and can be easily awoken. Your brain activity and eye movements slow down. Muscle activity also starts to relax. You spend a very short amount of time in stage 1 sleep.
Stage 2
During stage 2, eye movements stop completely. Brain waves become slower with occasional bursts of activity. Your body temperature drops and heart rate slows as you enter a state of deep relaxation.
Stage 3
Stage 3 is the deepest stage of non-REM sleep. It is also called slow wave sleep because the brain waves are very slow during this stage. Your muscles are fully relaxed, breathing is slower, and blood pressure drops. It is very difficult to wake someone from stage 3 sleep.
After deep non-REM sleep, we enter REM sleep, where most dreaming occurs. Brain wave activity increases and eyes move rapidly from side to side behind closed eyelids. Despite muscles being paralyzed during REM, heart rate and breathing quicken.
We cycle through these stages approximately 4-6 times per night. As morning approaches, we spend more time in lighter stages of sleep and REM sleep.
Hearing During Non-REM Sleep
So how does hearing work during these different stages of sleep?
During non-REM sleep, especially the deeper stages, our brains continue to process auditory information on some level. But the response is not the same as when we’re awake.
In stage 1 sleep, sounds may cause a slight arousal or make you shift your position. By stage 2, there is decreased awareness of external stimuli. Once stage 3 deep sleep sets in, even loud noises are unlikely to fully wake someone.
Research shows that when presented with sounds and noises during deep non-REM sleep, the brain responds by increasing certain brain waves, momentarily suppressing deep sleep to process the noise. But most of the time, sleep is not disrupted and conscious awareness does not occur.
However, very loud or familiar sounds can sometimes penetrate even stage 3 sleep. For example, a person may wake up to hearing their name called or a baby crying. This suggests we maintain some level of environmental monitoring and selectivity during deep sleep.
Hearing and Dreaming During REM Sleep
During REM sleep, our brain is highly active, nearly as much as when we’re awake. But we remain in a paralyzed state, unable to move our muscles voluntarily.
Hearing also behaves uniquely during the REM stage. The auditory system continues processing external sounds during REM. Research shows certain neurons in the brainstem become more receptive to acoustic stimulation specifically during REM sleep.
However, similar to deep non-REM, we rarely wake up in response to noises during REM sleep. Intense cognitive activity during dreaming may limit consciousness of external stimuli.
But sounds can often be incorporated into the dream narrative. Have you ever dreamt a ringing phone or alarm was part of the dream plot only to wake and realize it was real?
Some scientists propose external sounds are not only processed during REM but may actually influence dream contents by activating associated memories and emotions. This may explain why we dream about certain sounds heard while asleep.
Reasons We Can Hear During Sleep
The fact that we register and process sounds during all stages of sleep likely serves an evolutionary purpose. Here are some reasons why the ability is preserved:
Monitoring for threats
On some level, our brains remain alert to potential danger even as we sleep. Being able to wake up to significant or familiar sounds likely helped ensure safety and survival during evolution.
Assessing environment
Hearing what’s going on in our environment at night – even while asleep – helps calibrate circadian rhythms and sync sleep cycles to natural light/dark cycles.
Avoiding disruption
Having a high arousal threshold prevents constantly waking up to insignificant noises. At the same time, we remain receptive enough to wake if a sound signals a real need to become alert.
Consolidating memories
Stage 2 non-REM and REM sleep are when memory consolidation occurs. Being able to process sounds during these stages may help integrate auditory experience into long-term memory storage.
Factors That Impact Hearing During Sleep
While we maintain some level of hearing throughout the sleep cycle, certain factors affect auditory arousal thresholds and processing:
Sleep depth
The deeper the stage of non-REM sleep, the harder it is for a sound to cause awakening. Quiet sounds are unlikely to disturb deep sleep.
Sound type
Sudden, loud, or threatening noises are more likely to cause arousal than soft, steady, familiar sounds. Voices or music are less likely to awake someone than a smoke alarm.
Significance
Highly significant sounds like a baby crying or one’s own name can wake someone from even deep sleep since the brain recognizes the importance.
Sleep need
When sleep deprived, people may sleep more deeply and resist arousal to noises that would wake them with adequate sleep.
Health conditions
Deep sleepers or very heavy sleepers may have higher arousal thresholds to sounds. Certain medications, sleep disorders, and health issues can also impact responsiveness.
Alcohol
While alcohol may help induce sleep, it suppresses REM and causes more fitful sleep. This lowers arousal thresholds compared to sober sleep.
Improving Ability to Hear During Sleep
For heavy sleepers or those whose sleeping environment is noisy, these tips can help ensure you wake up to important sounds:
Use a loud alarm
Choose an alarm with an extra loud option or place it closer to your head to increase likelihood of waking up.
Try a vibrating alarm
For those who sleep very deeply, a vibrating alarm placed under your pillow or mattress can rouse you physically.
Minimize background noise
Use a white noise machine or earplugs to block constant low-level noise that could desensitize you to more urgent sounds.
Check medication effects
Discuss with your doctor if any medicines you take could alter arousal thresholds. Adjust timing or dosage to improve sleep quality.
Prioritize sleep schedule
Stick to a consistent bedtime routine and get at least 7-9 hours of sleep per night to avoid very deep deficit sleep.
Check hearing health
Rule out hearing loss, obstructions, or disorders that could physically prevent sounds from waking you.
Conclusion
Research shows we do continue to hear on some level during sleep, though not to the full degree as when awake. Responsiveness to sounds varies based on sleep stage, noise type, and significance. While we may not wake up to or remember all noises, the brain does continue processing auditory stimuli throughout the sleep cycle. This allows us to remain attuned to our surroundings for safety and circadian alignment while avoiding needless arousals. Understanding how hearing works during sleep provides insight into this fascinating aspect of human physiology.