It has long been thought that consuming caffeine can disrupt sleep hours later. Now a new study has found this to be true.

In fact, consuming caffeine even six hours before bedtime can have an effect, according to US researchers.

They monitored 12 healthy people who did not have any sleeping problems. All were told to maintain their regular sleeping schedule.

The participants were given three pills every day for four days. One pill was taken six hours before bedtime, one three hours before and the last as they were going to bed.

Two of the pills were a placebo, while the third contained 400mg of caffeine - this is equivalent to two to three cups of coffee. On one of the days, all three pills were a placebo.

Sleep disturbances were measured using a sleep monitor that could be used in the home and through sleep diaries.

The study found that consuming 400mg of caffeine at bedtime significantly disrupted sleep. However, it also found that consuming this much caffeine three or even six hours before bedtime also significantly disrupted sleep.

In fact, total sleep time among those who consumed caffeine even six hours prior to bedtime was reduced by more than one hour.

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

The researchers said that this is the first study to look at the effects of a specific dose of caffeine taken at different times before bedtime. They added that the results indicate that if people want to have a good night's sleep, they should not consume caffeine after 5pm.

Details of these findings are published in the Journal of Clinical Sleep Medicine.