Scientists say, your nightly cup of coffee may be keeping you awake for more reasons than you realise.
Research by Science Translation Medicine showed that caffeine was more than just a stimulant and actually slowed down the body's internal clock.
In the study a double espresso three hours before bedtime delayed the production of the sleep hormone melatonin by about 40 minutes, making getting to sleep tough.
One of the researchers included Dr John O'Neill from the Medical Research Council's Laboratory of Molecular Biology in Cambridge told the BBC News website: "If you're tired and having a coffee at night to stay awake, then that is a bad idea, you'll find it harder to got to sleep and get enough sleep."
As part of O'Neill's study, he grey cells in a dish and exposed them to caffeine to work out how it changed their ability to keep time. It showed the drug was able to alter the chemical clocks ticking away in every cell of the human body.
In an alternative study in the US, five people at the University of Colorado Boulder were locked in a sleep lab for 50 days, spending most of their time in very dim light, as light exposure is the main way in which we control our body clock.
The scientists discovered that an evening dose of caffeine slowed the body clock by 40 minutes - having a similar effect to three hours of bright light at bedtime.
Our sleep expert Dr Nerina advises to minimise your caffeine intake by drinking more water, herbal teas, and dilute fruit juices. She also says alcohol can impair deep sleep quality so you are likely to wake up feeling tired and fuzzy-headed if you have over indulged the night before.
Want more expert sleep opinions? Check out our very own Dr Nerina's sleep toolkit.