Getting between seven and eight hours of sleep each night in old age is the sweet spot for keeping your brain healthy, another study has found.
People who regularly got fewer than six hours had worse cognitive function and higher levels of a dangerous plaque in the brain linked to dementia.
And those who slept too much also performed poorer in memory, reaction time and flexible thinking tests, despite not having abnormal levels of amyloid beta.
The Stanford University study looked at nearly 4,500 people with an average age of 71 from the US, Canada, Australia and Japan.
Independent experts said it builds on existing research ‘highlighting a link’ between the build-up of amyloid protein and cognitive deterioration.
They added regularly sleeping too long in your senior years may be a symptom of underlying issues, such as depression.
Approximately one in three Britons and Americans do not get enough sleep each week, numerous surveys suggest.
Poor sleep has been linked to a host of conditions, such as heart disease, diabetes and high blood pressure, among others.
The NHS says most adults need between six and nine hours every night. It also urges people to try to wake up at the same time each day.
People who have trouble sleeping at night are significantly more likely to suffer a heart attack or stroke, scientists say
The above graph shows people divided by sleep duration (x axis) and levels of amyloid beta protein in the brain according to a PET scan (y axis). It shows that those that slept for six hours or less had the highest levels of amyloid beta protein. This has been associated with an increased risk of Alzheimer’s
The above graphs show three different measures of cognition. They reveal that people who slept for a normal amount of time scored highest on each of these tests
The new study, published in JAMA Neurology, included volunteers who were healthy and cognitively unimpaired.
Participants were asked to self-report how long they slept and take cognitive tests to assess their brain function.
Amyloid beta protein levels were measured using medical scans.
Experts cautioned that the study relied on participants self-reporting, which can be inaccurate.
They added it did not measure the quality of sleep in participants, and whether each went through the four stages of sleeping properly.
Of the participants, 1,185 were short sleepers (27 per cent), 283 were long sleepers (six per cent) and 2,949 were normal sleepers (67 per cent).
Results showed that long sleepers had the highest BMI (28.2), followed by short sleepers (27.8) and normal sleepers (27.3).
Cognition was assessed using several tests including a questionnaire handed to participants and their study partners, which scored them out of 15. A higher score suggested worse cognition.
Results showed people who slept for a normal period of time assessed themselves to have the best brain function (2.8) compared to short sleepers (3.3) and long sleepers (3.4).
Depression was measured using a separate questionnaire that does not ask about sleep disturbance, with higher scores indicating more depressive symptoms.
It found that people who slept for normal periods were least likely to be depressed, compared to short and long sleepers.
Beta amyloid protein levels in the brain were found to be highest in short sleepers.
The levels in normal sleepers were significantly different from this result, suggesting less sleep leads to more of this protein inside the brain.
Stanford neuroscientist Joe Winer, the lead scientist on the paper, said: ‘The patterns of cognitive performance were different between short and long sleepers.
‘Short sleepers did worse on memory and mini-mental state examination, consistent with early Alzheimer disease.
‘[But] long sleepers did worse on executive function, suggesting other (non-Alzheimer’s disease) processes.’
He added: ‘Our results are consistent with previous findings of both short and long sleep being associated with worse outcomes in late life.
‘I think our findings are exciting because they suggest short and long sleep duration might represent distinct [environmental influences] in aging.’
The head of communications at Alzheimer’s Research UK Dr Laura Phipps, who was not involved in the study, said it ‘adds to existing research highlighting a link between sleep patterns, the build-up of amyloid protein and cognitive ability’.
‘This new research is from a large, international study on cognitively healthy people, but it did rely on participants to report their sleep duration rather than measuring it directly.
‘The researchers couldn’t assess quality of sleep or time spent in different stages of a sleep cycle, each of which may be an important factor in the link between sleep and cognitive health.’
She added: ‘It is not clear why longer sleep duration is associated with negative health outcomes.
‘In this study, the researchers did not find a relationship between longer sleep duration and amyloid build-up in the brain but did find some evidence of a link with lower scores on certain cognitive tests, higher BMI and higher levels of depression.
‘It may be that longer than average sleep is a symptom of underlying issues, like depression, or a compensation for problems such as sleep apnoea, a breathing problem that can disrupt continuous, restful sleep.’
And she said: ‘The best evidence suggests that between seven and nine hours of sleep is optimal for most adults and anyone who thinks that their sleep patterns may be affecting their long-term health should speak to their doctor.’
The amyloid beta protein occurs naturally in the body and is made throughout the day to assist in brain function.
But at night while people sleep the protein is drained out of the brain.
If it is left to build up at high levels scientists say the protein can stick together and form clumps.
It is not clear what role it has in Alzheimer’s disease, but researchers think higher levels of the protein may disrupt communication between brain cells and eventually kill them.