Night-shift workers should avoid drinking coffee because it interferes with sleep and this side-effect worsens as people age, Canadian researchers said.

Julie Carrier, a University of Montreal psychology professor said the combined influence of age and caffeine made the sleep of middle-aged subjects particularly vulnerable to the circadian waking signal.

"Caffeine is the most widely used stimulant to counteract sleepiness, yet it has detrimental effects on the sleep of night-shift workers who must slumber during the day, just as their biological clock sends a strong wake-up signal," Carrier said in a statement. "The older you get, the more affected your sleep will be by coffee."

Twenty-four men and women participated in the study: one group was ages 20-30, while a second group was ages 45-60. All spent two sleepless nights in lab rooms before being allowed to sleep. Both participant groups took either 200 milligrams of caffeine or a lactose-based placebo.

The study, published in the journal Sleep Medicine, found the sleep of all subjects who consumed caffeine pills was affected, especially older participants who slept 50 percent less than usual. In both age groups, caffeine decreased sleep efficiency, sleep duration, slow-wave sleep and rapid eye movement sleep.