Scientists have developed a free app that tells you the ideal time to take a coffee break to keep you in the "optimal mental alertness zone".
The free Caffeine Zone iOS app, developed at Penn State University, is designed to help people determine when an extra jolt of caffeine may give them a mental boost and when it could harm sleeping patterns.
Users enter information about their caffeine consumption - how much caffeine they drank or plan to drink, when they consumed it and how fast - and the app combines this with data on the known effects of caffeine on the body to create a graph showing how caffeine will affect their system over time.
The green bar represents the optimal caffeine zone while the blue bar is where you'll need to get to before bed.
Drinking a cup of coffee rapidly gives people a spike in mental alertness but too much caffeine can also linger in the bloodstream and cause sleep problems hours later, said the researchers, who unveiled their app at the 2011 Augmented Cognition International Conference.
The app warns you if you're about to consume a coffee that will put you over the sleep threshold, while it also alerts you if you could do with one more cup.
"Many people don't understand how caffeine levels in their bloodstream go up and how they go down," said Frank Ritter, the university's professor of information sciences and technology, psychology and computer science and engineering.
"It's important to understand the effect that caffeine can have at these various levels."
According to the International Coffee Organisation, for roast and ground coffees the average amount of caffeine contained in a standard cup ranges from 80-115 milligrams, depending on whether you use the drip method or a percolator.
For instant coffees, the average amount of caffeine per cup is 65 milligrams. A can of Coke has 20-30mg of caffeine, a Red Bull has 80mg and a cup of tea has between 40-120mg.
The scientists said they used peer-reviewed studies to determine that caffeine drinkers are in an "optimal mental alertness zone" when they have between 200 and 400 milligrams of caffeine in their bloodstream. But going above this range can cause nausea and nervousness.
When they want to sleep, drinkers may discover that they have problems when they have more than 100 milligrams of caffeine in their bloodstream.
It takes about an hour for caffeine to reach its full effect in a normal adult and after 5-6 hours you will still have half the caffeine you consumed in your blood stream.
Ultimately, the researchers said the app was designed to help people determine when to modify their caffeine habits.
The study was supported by the US Office of Naval Research. Ritter said the app was especially useful for sailors, who have varying sleep patterns each day.
"If they, and others who drink coffee to stay awake, drink too much coffee on one shift, they may have trouble sleeping," said Ritter.
"So, the next day, they'll drink even more coffee and have even more trouble sleeping."
- Sydney Morning Herald
What's the most important thing about a home for you?
Hell is other parents
Pals and playmates (pictures)
New Zealand's best deck built yesterday
What do the stars have in store for you today?
Rev up your mind with our numbers game
Test your knowledge with our daily crossword