Drinking coffee before bedtime leads to the loss of an hour's sleep, according to a Wayne State University, Michigan, study.

For the study, the researchers analyzed the sleeping habits of 12 people over four days while they were given pills containing 400mg of caffeine.

Participants took one pill six hours before sleep, one at three hours before bedtime and the last one was taken right before they got into bed. Two of the three daily pills they consumed were placebos and on one day all the three were placebos.

The researchers found that the pills containing caffeine reduced their amount of sleep by more than hour. They now advise users to avoid drinking coffee after 5:00 pm.

"Drinking a big cup of coffee on the way home from work can lead to negative effects on sleep just as if someone were to consume caffeine closer to bedtime. People tend to be less likely to detect the disruptive effects of caffeine on sleep when taken in the afternoon," Psychiatrist Professor Christopher Drake, of Wayne State University said in a press release.

Researchers measured sleep disturbance subjectively with a sleep diary, and objectively using a home sleep monitor. This is the first study to find out the effects of caffeine taken at different times on sleeping habits.

"Sleep specialists have always suspected caffeine can disrupt sleep long after it is consumed. This study provides objective evidence supporting the general recommendation avoiding caffeine in the late afternoon and at night is beneficial for sleep," Dr Safwan Badr, of the American Academy of Sleep Medicine, said.

The finding has been published in the Journal of Clinical Sleep Medicine.