For many of us, a cup of strong coffee is just the thing to jolt us into action in the morning.
And with Britons drinking 95million cups a day, it has never been more popular.
But now scientists have discovered that a lifetime of daily coffees may shrink a part of the brain that controls sleep patterns.
They think the findings may partly explain why some older people struggle to drop off.
Brain scans carried out by researchers at Seoul University in South Korea found moderate to heavy consumers – those drinking two cups a day for 30 years or more – had smaller pineal glands than those who rarely drank coffee.
The pineal gland is a pea-sized organ in the middle of the brain that releases a hormone called melatonin when it is time for the body to rest and sleep.
The smaller the gland is, the less melatonin it produces.
Although caffeine is well known as a short-term stimulant, this is thought to be one of the first studies to suggest it could have long-term effects on the brain.
Researchers tracked 162 elderly healthy men and women and quizzed them on how much coffee they drank and how long they slept.
They then performed MRI brain scans to measure the volume of the pineal gland.
They found that coffee-lovers had pineal glands 20 per cent smaller than non-drinkers and experienced more problems sleeping.
They said that years of daily coffee drinking could be harming the brain and the quality of sleep later in life.
In their report, published in the journal Sleep, the scientists warned: ‘Given the large amount of coffee consumption across the world and the rapid increase in the use of caffeine in children and adolescents over the past 30 years, we should be concerned with the potential adverse effects of lifetime coffee consumption.’
However, Dr Neil Stanley, an independent sleep expert, said the research did not prove caffeine in coffee was harming sleep quality among the elderly.
He added: ‘Different cups of coffee can have different levels of caffeine in them. But the study did not take this into account, nor any other sources of caffeine that they may have consumed over the years.’