Taking caffeine even six hours before bedtime could reduce sleep by more than one hour, an effect that's underestimated by the general public and doctors, researchers say.

While sleep experts recommend refraining from drinking coffee or consuming other food and drinks with caffeine close to bedtime, few studies have checked into the effects of taking caffeine in the late afternoon and early evening.

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The caffeine content of beverages and foods is increasing, studies suggest. (Tom Hanson/Canadian Press)

 A small study published in Friday's Journal of Clinical Sleep Medicine tested the effects on sleep of 400 milligrams of caffeine — about 2-3 cups of coffee — taken at bedtime, three and six hours before lights out.

"Even at six hours, caffeine reduced sleep by more than one hour," Christopher Drake of the Henry Ford Sleep Disorders and Research Center in Detroit and his co-authors concluded. Losing an hour of sleep over multiple nights may harm daytime function, the researchers said.

"The present results suggest the common practice of afternoon consumption of caffeine should at a minimum be restricted to before 17:00, particularly with regard to the moderate-large doses of caffeine commonly found in increasingly popular premium coffees and energy drinks." Commercially available 16-ounce servings of specialty coffee were found to contain up to 500 milligrams of caffeine in one study.

Health Canada recommends no more than 400 milligrams of caffeine per day — about three 8 ounce cups (237 ml) of brewed coffee  — with a lower limit for women who are pregnant, breastfeeding or planning to become pregnant. Drake's team tested 12 healthy people with an average age of 39 who were asked to continue their normal sleep schedules of going to bed between 21:00 and 01:00 and waking between 0600 and 0900.

The subjects were given three pills a day for four days and told to take one pill at six, three and zero hours before bedtime. There were 400 milligrams of caffeine in one of the pills and the other two were placebos. On one of the days, all three pills were placebos. There was also a washout night before each pill day.

The subjects filled in sleep diaries and had their sleep disturbances measured objectively using an in-home sleep monitor that records EEG signals.

The participants noticed their sleep was disturbed when they took caffeine pills at bedtime or three hours earlier but not six hours before. The researchers called the lack of perceived sleep disturbance an important finding that points to the need for more education.

"The risks of caffeine use in terms of sleep disturbance are underestimated by both the general population and physicians," the study's authors said.

Limitations of the study include the small number of subjects and lack of measuring caffeine levels in the blood.

The study was funded by Zeo Inc. and several authors have financial interests or served as consultants to drug and sleep device manufacturers.