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Daytime exposure to bright light may help improve sleep and relieve some dementia symptoms in older people by strengthening their circadian rhythms. These 24-hour biological cycles enable our bodies to switch tasks and recuperate by synchronising specific activities, including sleep, with day and night.

As we age, the lenses of our eyes grow cloudier, meaning less light reaches photoreceptive cells in the retina that connect to the body鈥檚 master clock in the brain, called the suprachiasmatic nucleus. That can be problematic, as daylight helps keep our biological rhythms in line with external time.

It is even worse for elderly care home residents who rarely venture outside, and who are often exposed to light at night, to help keep them safe.

鈥淲hen people are in constant dim light, their bodies don鈥檛 know what day and night is,鈥� says Mariana Figueiro at the Lighting Research Center in New York. Many elderly care home residents experience insomnia and disrupted circadian rhythms, which may present as excessive daytime sleepiness, nocturnal wandering and sundowning 鈥� agitation and irritability during late afternoon and early evening.

Previous research has suggested that boosting light levels during daylight hours might reduce the rate of cognitive deterioration among care home residents with dementia. However, others have tried to replicate these findings with mixed results.

One problem is ensuring that enough light is delivered into people鈥檚 eyes. Since many care home residents with dementia spend their daytimes seated in communal areas, Figueiro and her colleagues designed a light table that directs light upwards.

These were installed in eight US care homes, along with additional light boxes and floor lamps to deliver lighting that was bright enough and contained enough blue-spectrum light to activate the circadian system. They were used by 46 residents with moderate to late-stage dementia for four weeks.

These experimental lights were switched on from when the residents woke up each morning until 6 pm, after which the normal care home lights continued to be used. During an additional four-week period, the participants used the table with dimmer lights with more of a yellow hue that wouldn鈥檛 activate the circadian system.

Compared with the yellower light, or no supplemental light, the bluer lighting intervention significantly improved sleep quality, and reduced depressive symptoms and agitation, as assessed by the residents鈥� caregivers. 鈥淭hey also told us that they were eating better, they were more social and they were easier to deal with,鈥� says Figueiro. 鈥淢y hypothesis is that they are getting better sleep, and this impacts their agitation and mood.鈥�

鈥淭he main limitation is that the questionnaires were completed by the carers, not the persons with dementia,鈥� says Derk-Jan Dijk, director of the Surrey Sleep Research Centre in the UK. It may be difficult to accurately assess agitation levels on behalf of someone else, though there were clear improvements in sleep quality and depression symptoms, he says.

Even so, this builds on growing evidence that daylight exposure may be important for improving sleep in this population, he adds. Dijk is currently working with the UK Dementia Research Institute to develop and test new technologies to improve sleep and circadian rhythms in people with dementia.