At least five hours sleep a night may cut the over-50s' chances of multiple chronic health problems, researchers say.
Ill health can disrupt sleep – but poor sleep may also be a forewarning or a risk itself, they say.
There is evidence sleep helps restore, rest and rejuvenate the body and mind – but why the "golden slumber number" might matter remains unclear.
The PLoS Medicine study tracked the health and sleep of UK civil servants.
All of the about 8,000 participants were asked: How many hours of sleep do you have on an average weeknight?"
Some also wore a wrist-watch sleep tracker.
And they were checked for chronic conditions, including diabetes, cancer and heart disease, over two decades of follow-up:
Experts generally recommend about seven or eight hours, the researchers, from University College London and Paris Cité University, say.
Scientists do not know for sure, but it is clear that sleep helps the brain process memories and is good for mood, concentration and metabolism.
Sleep is also an opportunity for the brain to be cleared of waste.
Surrey Sleep Centre director Prof Derk-Jan Dijk told BBC News: "This work reinforces that getting only short sleep is not good for us. Generally, it's not healthy – although for some, it may be OK.
"The big question is why do some people sleep less. What is causing it and is there anything we can do about it? Sleep is a modifiable lifestyle factor to a certain extent."
Long stretches of bad sleep can severely affect wellbeing.
GPs now rarely prescribe sleeping pills, which can have serious side-effects and cause dependency.
But sleep problems can often be resolved and support is available.
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