From VOA Learning English, this is the Health & Lifestyle report.

Caffeine is the most widely used drug in the world. Many people drink coffee or tea to help them wake in the morning. And many others drink caffeine throughout the day to prevent sleepiness.

So, it is no surprise that if you get too much caffeine before bedtime, it can keep you awake. It turns out that interrupting your sleep is bad for your health on many levels, including the cellular level.

A new study explains how interrupted sleep can affect your mental and physical health. The study also explores how poor sleep can affect the cells in your body.

Researchers from the United States and the United Kingdom have joined from across the pond to investigate this issue.

The investigators are from the University of Colorado, Boulder in the U.S. and the Medical Research Council Laboratory of Molecular Biology in Cambridge, England. They found that caffeinated drinks taken up to three hours before expected bedtime can delay normal sleep times by as much as 40 minutes.

The amount of caffeine that disrupted sleep was equal to about two shots of espresso.

Kenneth Wright is head of the Sleep and Chronobiology Laboratory at the University of Colorado Boulder. Mr. Wright warns some coffee drinks sold in coffee shops usually contain more caffeine than that.  

"This particular finding tells us that the timing of sleep and wakefulness will be pushed later because of an effect on the (biological) clock, not just promoting wakefulness chemicals in the brain."

This is no surprise. Scientists have known for a long time that caffeine keeps you awake. Caffeine affects the chemicals in the brain that control wakefulness. Caffeine also blocks chemicals in the brain that promote, or cause you to sleep.

And interrupted sleep is not good for you.

Not getting enough sleep can affect a person’s mood. It also disrupts the body’s natural sleep-wake cycle called the circadian rhythm. Your body’s circadian rhythm is found in cells throughout the entire body.

So, when caffeine disrupts your circadian rhythm, it affects hormone production and the re-growth of new cells in the human body. And this can play a role in many health problems – from obesity to diabetes to cancer.

The experiment

To look into caffeine's effect on the circadian rhythm, or circadian clock, researchers first noted the sleep-wake cycles of five healthy volunteers.

The study lasted 49 days. Researchers gave the participants 200 milligrams of caffeine a few hours before bed. Two hundred milligrams of caffeine is about the amount found in two shots of espresso. Then the researchers noted how long it took the volunteers to fall asleep.

The volunteers were also exposed at night to bright light, which is also known to disrupt sleep. Caffeine, however, interrupted the circadian clock, and patterns of sleep, more so than bright light did. Caffeine affects the production of melatonin, the sleep-producing hormone.

When a person’s circadian rhythm is disrupted, sleeping and eating patterns are thrown off. A disrupted circadian rhythm increases the chance of heart disease, obesity and mental illnesses such as depression and bipolar disorder. Bipolar disorder is a disease of the brain where people experience extreme highs and extreme lows.

But caffeine’s effect on the body is not all bad.

Mr. Wright says people could use caffeine to help their body’s clock, for example, when they travel.

"Another example of an implication of our findings is we may be able to use caffeine to help shift our clocks westward when we're traveling across many time zones. In this case here, caffeine may help us adapt to jet lag must faster."

The scientists published their findings in the journal Science Translational Medicine. The scientists also give some common sense advice -- people who want to wake up earlier in the morning might want to avoid that nighttime cup of caffeine.

And that’s the Health & Lifestyle report.

I’m Anna Matteo.

Do you drink caffeine before bedtime and does it affect your sleep or mood? Let us know in the comments section and on our Facebook page.

Jessica Berman reported this story from Washington, D.C. for VOA News. Anna Matteo adapted it for Learning English. Caty Weaver was the editor.

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interrupt – v. to cause (something) to stop happening for a time

caffeine – n. a substance that is found especially in coffee and tea and that makes you feel more awake

across the pond – idiomatic phrase : that means on the other side of the Atlantic Ocean. It is used to refer to the United States or the United Kingdom depending on the speaker's location

disrupt – v. to cause (something) to be unable to continue in the normal way

circadian rhythm – n. daily rhythmic activity cycle, based on 24-hour intervals, that is exhibited by many organisms

promote ­ – v. to help (something) happen, develop, or increase

hormone – n. a natural substance that is produced in the body and that influences the way the body grows or develops

depression – n. a serious medical condition in which a person feels very sad, hopeless, and unimportant and often is unable to live in a normal way

bipolar disorder – n. any of several psychological disorders of mood characterized usually by alternating episodes of depression and mania —called also manic depression, manic-depressive illness