Why it's so hard to get off to sleep

Scientists claim to have discovered why some people can shut themselves off to the world after falling asleep while others are awakened by the slightest sound.

Researchers in the US believe a sensory gateway in the brain called the thalamus plays a key role in blocking out sound during sleep. The effect can be seen in brief bursts of electrical activity generated by the brains of sleepers, known as "sleep spindles".

"The more sleep spindles your brain produces, the more likely you'll stay asleep, even when confronted by noise," said Dr Jeffrey Ellenbogen, of Harvard Medical School in Boston.

Boosting sleep spindles through behavioural techniques, drugs or electronic devices might help light sleepers to have a restful night – although how exactly is not yet clear.

The effect was seen in recordings of the brainwave patterns of 12 volunteers asked to endure noisy nights in a sleep laboratory. On the first night they were allowed to sleep undisturbed, but over the next two nights they were subjected to an array of noises, including a telephone ringing and people talking.

People who produced more sleep spindles on the quiet night were better able to tolerate the noisy nights.