If you have ever spent a night in a hotel or sleeping on a friend’s couch, you probably understand how much of an impact the environment around us can have on how well we sleep. Lumpy couches, excess outside illumination, and the weird sounds of “other” places can keep us from getting the sleep we need when we are away from home. The same thing can happen in the comfort and familiarity of our own bedrooms. While we are much less likely to experience sleep disruptions caused by comfort, temperature, and light, since these are factors we can more easily control, expected noises like the phone ringing or a neighbor’s dog barking can still keep us from getting the sleep we need.
However, there are some people who seem to be able to sleep through anything, including these sounds. This fact intrigued researchers from Massachusetts General Hospital and Harvard Medical School who wanted to understand what made some sleepers sleep soundly while others struggled to sleep because of sound. In order to understand what made the sound sleepers different, the research team went in search of a biological basis that enabled some people’s sleep to be more impervious to sound.
The team conducted a study that included 12 participants who were in good health. Each participant spent three nights sleeping at a sleep lab so that their sleep could be monitored under differing conditions. Using electroencephalography (EEG), the team studied the brain waves of the participants during the different phases of sleep to see if there were demonstrable differences between those whose sleep is disrupted by external sounds and those whose sleep is not. For the first night, the participants were monitored while their environment was quiet. This provided a kind of baseline for the team to use for comparison with the other two nights where the participants were frequently exposed to disruptive sounds.
Previous research results indicated that there may be a relationship between something called sleep spindles and the brain’s ability to block sensory input during sleep. In order to understand how this relationship might work, it is helpful to understand a little more about how our brain deals with sensory input, like sound, while we are asleep. Our brain receives all sensory information into the cortex for processing via the thalamus. The connection and communication between these two parts of the brain continue during sleep and can be seen occurring on the EEG. This means that if the sensory input of a loud sound, like the phone ringing, reaches your thalamus while you are sleeping, your brain continues processing it as if you were awake. This is why sound can be disruptive to sleep.
Sleep spindles are brain activity that only occurs when you are sleeping. These spikes in brain activity occur during the middle of the sleep cycle, in the second and third stage of non-REM sleep. In order to understand why some people can sleep soundly regardless of external noise and some cannot, the research team wanted to see if there was any correlation between these sleep spindles, which are thought to block sensory input from reaching the thalamus during sleep, and the sound sleepers.
The findings, which were posted in the journal Current Biology, were significant. On the second night and third nights, the participants were exposed to common disruptive sounds like a phone ringing. Across the participants, those who were able to sleep soundly all three nights showed a higher incidence of sleep spindles than those whose sleep was disrupted. Although the size of the study was small, the results may pave the way for additional research that expands our understanding of how sleep is disrupted and the natural ways the brain works to help us stay asleep.