Dementia could be slowed significantly by treatments which reset the body's natural clock, researchers have said.
The Dutch team used brighter daytime lighting - with or without the drug melatonin - to improve patients' sleep, mood and cut aggressive behaviour.
It concludes that these can slow deterioration by 5% - which a UK specialist said meant patients living in their own homes for months longer.
The study appears in the Journal of the American Medical Association.
"If someone could be kept at home for an extra six months, rather than placed in a care home, there are huge personal and social benefits"
The disruption to the body's circadian rhythm - the natural cycle that governs sleep and wakefulness - can be one of the most difficult of dementia symptoms for carers to cope with.
It can mean that people with the illness can be asleep during the day, but fully awake for periods during the night.
Other studies have suggested that the use of bright room lighting and melatonin can help adjust the "clock", and the researchers from the Royal Netherlands Academy of Arts and Sciences in Amsterdam managed to recruit 189 care home residents to take part in an unique trial.
Six of the care homes taking part had lighting installed, and this was turned on between 9am and 6pm every day.
Some of the patients, most of whom had some form of dementia, received melatonin, a naturally-occurring hormone, and their progress was then monitored for at least the next year.
Those who had melatonin, but no extra lighting, had better sleep patterns, but tended to be more withdrawn and have a worse mood.
However, patients having melatonin and bright light together managed to avoid these mood problems.
Even having the light without melatonin slowed "cognitive deterioration" by 5% compared with those homes which did not install brighter lighting, and depressive symptoms fell by 19%.
'Spectacular'
The study authors said that care homes should consider introducing the lights for their residents with dementia.
Dr Michael Hastings, from the Medical Research Council Laboratory for Molecular Biology in Cambridge, and himself a researcher into circadian rhythms and Alzheimer's disease, said the study results were "spectacular".
"Although 5% may not sound like a huge amount, it compares well with treatments such as Aricept designed to slow the progression of the illness.
"Over the course of Alzheimer's, it could represent six months, and you have to remember that the light therapy is completely non-invasive, and melatonin is a very gentle drug."
He said that sleep disturbances were often the "final straw" for relatives trying to cope for people with dementia.
"You can have a situation where someone is asleep for part of the day, then at 3am will be awake, wandering around the house, turning the gas on. Relatives can manage quite a few of the symptoms of mild or moderate dementia, but this can be too much.
"It's a crunch issue, and if someone could be kept at home for an extra six months, rather than placed in a care home, there are huge personal and social benefits."
He added that since circadian rhythm disruption was a feature of other neurological diseases, such as Huntington's and Parkinson's, there might also be an application for the therapy elsewhere.
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