Low levels of a particular chemical during the night may be important in the formation of new memories.
Alzheimer's drugs boost acetylcholine levels
Researchers from a German University found that volunteers with boosted levels of acetylcholine performed less well in night-time memory tests.
The study, published in Proceedings of the National Academy of Sciences, is intriguing for experts interested in Alzheimer's disease.
A lack of the chemical is currently thought to play a role in the illness.
The small study at the University of Lubeck focuses on healthy young men - rather than older subjects or Alzheimer's patients.
The researchers wanted to test the relationship between levels of the brain chemical to their ability to lay down new memories during the night.
Scientists believe that periods of sleep called short-wave sleep are important for the consolidation of "memories" acquired during the day.
Just before going to bed, some of the volunteers were given a drug designed to increase levels of acetylcholine, which is a chemical neurotransmitter found in the brain and central nervous system.
The others were left with normal levels of acetylcholine.
In the middle of the night, they were re-tested on a list of words they had been asked to memorise the previous day.
Those with high acetylcholine levels did worse than those who had not been given the drug.
While the aim of the study was primarily to test the mechanisms of memory storage, the authors said it might suggest ways of improving the treatment of some Alzheimer's patients.
Many take drugs that boost acetylcholine levels before going to bed because they will then sleep through any unpleasant side-effects.
However, the researchers wrote: "The finding implies that the administration of cholinesterase inhibitors before sleep in Alzheimer's patients should be reconsidered."
However, Professor Gordon Wilcock from the Bristol Dementia Research Group told BBC News Online that many patients achieved benefits from taking these drugs prior to going to sleep at night.
He said that it was difficult to draw conclusions about acetylcholine levels in elderly Alzheimer's patients, who may be deficient in the chemical, from tests on younger, healthy volunteers.
He said: "It's possible that giving the drug to a young person who is producing enough acetylcholine ends up overloading the brain.
"We know that a significant number of dementia patients benefit from using these drugs - although there are some who achieve no benefits at all."