Low levels of electrical activity in the brain may cause some people with epilepsy to have seizures, say experts.
Many people with epilepsy have seizures after falling asleep
A team of international scientists carried out tests on 14 people with epilepsy and two without.
They found that activity in the outer part of the brain slowed significantly when those with epilepsy were asleep.
Writing in the Proceedings of the National Academy of Sciences, they said it may explain why many have seizures after falling asleep.
Doctors have known for many years that sleep can trigger epileptic seizures. However, they have been unable to explain exactly why this happens.
Previous studies have suggested it may be linked to very slow electrical activity in the brain.
However, scientists have been unable to confirm this theory, largely because they were unable to detect very slow brain waves using conventional machines.
But now scientists at the University of Helsinki and the University of Washington have developed a highly sensitive electroencephalography (EEG) machine to pick up these waves.
They carried out tests on each of the 16 volunteers while they were asleep over the course of nine days.
They found that brain activity slowed considerably in those with epilepsy. But they also found that this was quickly followed by sharp bursts in brain activity.
These sharp bursts, which are known as interictal epileptic events, are a characteristic of epilepsy.
The scientists said further research is needed to examine this link.
But they said the finding could potentially lead to new treatments for people with epilepsy.
Professor John Duncan, medical director of the National Society for Epilepsy, welcomed the study.
"This is potentially interesting area of EEG research, with application for epilepsy, as it may show new ways of analysing the EEG that give more information about the liability to have seizures, and also to predict when seizures are going to occur."
But he added: "It is a long way from clinical maturity though."