While you sleep, your brain continues to register and process sounds on a basic level. Noise can jostle your slumber—causing you to wake, move, shift between stages of sleep, or experience a change in heart rate and blood pressure—so briefly that you don’t remember the next morning. Whether sounds disturb your sleep depends on factors such as the stage of sleep you’re in, the time of night, and even your feelings about the sounds themselves.
Noises are more likely to wake you from a light sleep (stages 1 and 2), than from a deep sleep (stages 3 and 4), and tend to be more disruptive in the second half of the night. If you share a bed with someone, you know that there is individual variation in sensitivity to noise. In fact, a recent study found that “sound sleepers” have characteristic brain activity that may make them more impervious to noise.
Interestingly, whether or not a sound bothers your sleep depends in part on that sound’s personal meaning: researchers have seen that people are more likely to wake when a sound is relevant or emotionally charged. This is why, for example, a parent could sleep soundly through her partner’s snores but wake fully when her baby fusses.