A team of Dutch scientists are suggesting that the progression of dementia can be significantly slowed down by treatments which reset the body's natural clock.
Mood swings, sleep problems and behavioural issues often affect those with dementia-related cognitive decline and the researchers say brighter daytime lighting improves these symptoms.
The scientists say when brighter daytime lighting is used, with or without the drug melatonin, the deterioration in mental faculties can be slowed down by 5%.
Experts say this means patients could continue to live in their own homes for months longer with huge personal and social benefits.
They say the disruption to the body's circadian rhythm which controls the natural cycle of sleep and wakefulness, can be one of the most difficult dementia symptoms for carers to deal with.
Dementia sufferers are can often be asleep during the day, but fully awake for periods during the night and other research has also suggested that the use of bright room lighting and melatonin can help adjust the body clock.
The researchers from the Royal Netherlands Academy of Arts and Sciences in Amsterdam recruited 189 care home residents, at 12 elder care facilities.
In the trial the participants were on average 86 years old on average and mostly female.
It was organised for six of the care homes involved to have lighting installed which was switched on between 9am and 6pm every day, and some increased the intensity of their ceiling-mounted lights.
Most of the patients had some form of dementia and some were given the hormone melatonin, which occurs naturally in the body and were then monitored for a period of at least one year.
It was found that those who were given the melatonin, but no extra lighting, had better sleep patterns, but tended to be more withdrawn and have worse moods but patients receiving both the melatonin and bright light managed to avoid the mood problems.
The researchers led by Dr. Rixt F. Riemersma-van der Lek, say 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%.
Experts have called the study results "spectacular" and say while 5% may not sound a huge amount, it compares well with drugs such as Aricept which are designed to slow the progression of the illness.
They say for relatives trying to cope with people with dementia, mild or moderate symptoms of dementia can be dealt with but sleep disturbances where people are wandering around in the middle of the night, were often the last straw.
Circadian rhythm disruption is also a feature of other neurological diseases, such as Huntington's and Parkinson's, and the experts say the light therapy there might also be useful in these areas - also the light therapy is completely non-invasive, and melatonin is a very mild drug.
The study authors said that care homes should consider introducing the lights for their residents with dementia.
The study is published in the Journal of the American Medical Association.