The 'sleep spindles' that help us wind down
Now scientists believe they have identified a sensory gateway in the brain which plays a key role in blocking out sound during sleep.
The effect can be seen in brief bursts of electrical activity generated in the brain of sleepers, known as "sleep spindles".
Dr Jeffrey Ellenbogen, a researcher from Harvard Medical School in Boston, said: "The more sleep spindles your brain produces, the more likely you'll stay asleep, even when confronted by noise."
Boosting sleep spindles through behavioural techniques, drugs or electronic devices could help light sleepers have a restful night, the scientists believe, although how this can be achieved is not yet clear.
The effect was seen in recordings of the brain wave patterns of 12 volunteers who underwent laboratory sleep experiments - one night in quiet surroundings followed by two nights of sleep disruption from noise including a telephone ringing, people talking, and mechanical sounds of the sort frequently heard in hospitals.
During sleep, brain waves become slow and organised, said Dr Ellenbogen. Sleep spindles are brief bursts of faster-frequency waves which stand out against the calm background.
They are generated by the thalamus, a "checkpoint" in the brain through which all kinds of sensory information pass, apart from smell signals, said the scientists.
Volunteers who produced more sleep spindles during the quiet night were better able to tolerate the subsequent noisy nights, the study found.
"The thalamus is likely preventing sensory information from getting to areas of the brain that perceive and react to sound," said Dr Ellenbogen.
"Our data provides evidence that the sleep spindle is a marker of this blockade. More spindles means more stable sleep, even when confronted with noise.
"The effect of sleep spindles was so pronounced that we could see it even after just a single night."
Ways of enhancing the sleep spindle effect would be welcome in an increasingly noisy modern world, the researchers believe.
Dr Simon Kyle, of the Glasgow Sleep Centre, said: "This is an important, novel finding which could provide further understanding of individuals' vulnerability to sound during sleep.
"The interesting thing is that a person's sleep spindles are a highly stable trait.
"Obviously this is ongoing research, but if the spindle rate could be boosted it may well have benefits for those suffering from insomnia. It could also help older people, as spindle rate decreases with age.
"The reasons why some people have a lower spindle rate has not been systematically examined, so research looking at ways of giving an extra rate of protection against noise is to be welcomed."
The findings are published in the journal Current Biology.