Night-time noise remains a nuisance

"Scientists in the US have cracked the secrets of heavy sleepers – and their findings could help those of us who are woken more easily," reports the Daily Express.

This news story is based on a small study that measured the brain activity of 12 volunteers over three consecutive nights to find out if a certain type of brain activity – called sleep spindles – play a role in blocking out noise during sleep. It suggests that people with a higher rate of sleep spindles could withstand greater exposure to noise without waking.

The researchers suggest that their finding could be used to develop treatments to protect sleep from being disrupted by noise. However, this is a very preliminary finding and the possibility of treatment based on this very small study is a long way off. Furthermore, noise is only one factor that can affect sleep. Other factors include anxiety, depression, alcohol and poor sleep routines.

Where did the story come from?

US researchers from Harvard Medical School, Massachusetts General Hospital, Brigham and Women’s Hospital and Cambridge Health Alliance carried out the study, along with researchers from the University of Liege in Belgium.

The study was published as a letter in the peer-reviewed journal Current Biology . It was funded by the Academy of Architecture for Health, the Facilities Guidelines Institute, the Centre for Health Design and the Massachusetts General Hospital.

The study was widely reported in the media. Most of the reporting was fair, although the importance of these findings and their implications for treatment of insomnia were generally exaggerated. For example, the BBC said that scientists had discovered the clue to getting a good night’s sleep, when noise is only one factor that can disrupt sleep.

What kind of research was this?

This small laboratory study investigated whether a certain pattern of electrical activity in the brain affects a person’s ability to sleep through noise. The researchers say the occurrence of this electrical activity, called sleep spindles, can vary between people, but is stable across nights. The spindles are thought to hinder the transmission of external stimuli, such as noise, from the thalamus to the cortex, blocking out sound and preserving “sleep stability”. The researchers aimed to test the hypothesis that people who generate more spindles would need louder sounds to disrupt their sleep.

The study did not look at other factors that might affect a person’s sleep, so it cannot definitively conclude that these sleep spindles are connected to sleep stability. It can only show an association between spindle rate and sleep stability (defined as the maintenance of sleep without arousal). Also, it measured sleep stability with an EEG test (electroencephalogram – a method of recording electrical activity produced by the firing of neurons in the brain), rather than looking at whether individuals themselves reported sleep problems.

What did the research involve?

The researchers studied 12 healthy volunteers with an average age of 26 in a sleep laboratory for three consecutive nights. The first night was quiet while the second and third were noisy, with the researchers using common sounds during the different stages of sleep, such as road traffic and a phone.

Brain activity was monitored each night throughout the different stages of sleep with an EEG. These stages are broadly characterised as either REM (rapid eye movement) or non-REM sleep. The majority of sleep is non-REM, of which there are descending levels of brain activity until it reaches its lowest level. REM sleep is the stage when the brain is most active and dreams are thought to occur.

The researchers say they used the first “quiet” night’s measurements to calculate each of the volunteer’s normal “spindle rates”, during non-REM sleep as the spindle pattern only occurs during this stage. They calculated each subject’s spindle rate as the number of detected events per minute on the EEG during the second and third stages of non-REM sleep.

On the second and third nights, noises were introduced during both non-REM and REM sleep. The noises lasted for a period of 10 seconds, starting at 40 decibels and increased gradually in volume every 30 seconds until sleep was disturbed (measured by EEG and using standard guidelines for defining sleep arousal).

The researchers then looked at the relationship between individual spindle rates and people’s sleep stability, using standard statistical methods.

What were the basic results?

The researchers found that people with higher spindle rates on the quiet night had higher tolerance of noise during the subsequent noisy nights.

More than half the volunteers with high spindle rates had stable sleep compared to less than half of those with lower spindle rates at the 40dB sound levels.

How did the researchers interpret the results?

The researchers say it is possible to predict an individual’s ability to maintain sleep despite external noise and that those with higher spindle rates are more resistant to sounds during sleep. They also speculate that this finding could explain an association found in previous studies between spindle rate and people’s learning potential. They say that if spindles shield sleep from disruption they may allow certain brain processes to be consolidated.

The data, they say, raises questions as to whether treatments could be developed that enhance spindle rates and therefore help to protect sleep.

Conclusion

This laboratory study found that a certain type of brain activity is associated with rates of sleep stability as measured by EEG, in a relatively young age group. These findings may be of great interest to sleep scientists, but there are a few limitations in relation to its bearing on treatments for sleep disorders.

The researchers did not look at other factors that disturb sleep, nor did they look at whether self-reported sleep problems were related to spindle rates.

In addition, the study was in a small group of healthy young adults and the findings may not apply to other populations, such as people with sleep problems or older people. As the researchers point out, both noise tolerance and spindle rate diminish with age, and elderly people are thought to be particularly vulnerable to sleep problems. Further research will need to answer these questions.