Seven hours of sleep per night is the ideal amount in middle to older age, a study suggests.

Both too much and too little sleep are associated with poorer cognitive performance and mental health, according to researchers from Cambridge University and Fudan University in China.

Scientists examined data from nearly 500,000 adults, aged between 38 and 73, from the UK Biobank.

Participants were asked about their sleeping patterns, mental health and wellbeing, and took part in a series of cognitive tests.

Brain imaging and genetic data were available for almost 40,000 of the study participants.

The researchers' analysis of the data indicated that seven hours of sleep per night was the optimal amount for cognitive performance, such as processing speed, visual attention, memory and problem-solving skills.

It was also optimal for good mental health, with people experiencing more symptoms of anxiety and depression, and worse overall wellbeing, if they reported sleeping for longer or shorter amounts.

The researchers said one possible reason for the association between insufficient sleep and cognitive decline may be due to the disruption of slow-wave or "deep" sleep.

Disruption to this type of sleep has been shown to have a close link with memory consolidation as well as the build-up of amyloid - a key protein which, when it misfolds, can cause "tangles" in the brain characteristic of some forms of dementia.

Additionally, lack of sleep may hamper the brain's ability to rid itself of toxins.

"While we can't say conclusively that too little or too much sleep causes cognitive problems, our analysis looking at individuals over a longer period of time appears to support this idea," Professor Jianfeng Feng, from Fudan University, said.

The researchers said the findings suggest insufficient or excessive sleep duration may be a risk factor for cognitive decline in ageing.

This is supported by previous studies that have reported a link between sleep duration and the risk of developing Alzheimer's disease and dementia, in which cognitive decline is a hallmark symptom.

"Getting a good night's sleep is important at all stages of life, but particularly as we age," Professor Barbara Sahakian, from Cambridge University's Department of Psychiatry, said.

"Finding ways to improve sleep for older people could be crucial to helping them maintain good mental health and wellbeing, and avoiding cognitive decline, particularly for patients with psychiatric disorders and dementias."