People who work night shifts should give up drinking coffee if they want to get some decent sleep between shifts, suggests a new study from Canadian researchers.

In fact, older people in general should cut out java to improve their sleep, the study authors conclude.

The study, published in the journal Sleep Medicine, notes that sleeping during the day is already difficult since we are working against our internal clock, called our circadian rhythm. But the caffeine in coffee just worsens that and further interferes with sleep, the research led by Julie Carrier, a Universit� de Montr�al psychology professor and a researcher at the affiliated H�pital du Sacr�-Cœur Sleep Disorders Centre, found.

"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," says Carrier.

What's more, this side-effect worsens as people age, the study found.

"The older you get, the more affected your sleep will be by coffee," Carrier says.

For the study, 24 men and women were brought to a sleep clinic: one group was aged 20 to 30, while a second group was aged 45 to 60.

Everyone spent two sleepless nights in lab rooms before being allowed to sleep.

Both participant groups had to take a pill three hours before sleeping; either 200 milligrams of caffeine or a lactose-based placebo.

Those participants who had the caffeine pills had their sleep affected. In both age groups, caffeine decreased sleep efficiency, sleep duration, slow-wave sleep (SWS) and REM sleep.

The effect was even more pronounced in the older participants, who slept 50 per cent less than usual.

The combined influence of age and caffeine made the sleep of middle-aged subjects particularly vulnerable to the circadian waking signal, Carrier explains. She suggests that lower brain synchronization - caused by age and caffeine - produces greater difficulty in overriding circadian waking signals during daytime and that leads to fragmented sleep.

Carrier adds that while there are plenty of people who say they can sleep just fine even after drinking espresso, this study finds that's likely not true.

"Although they may not notice it, their sleep will not be as deep and will likely be more perturbed," she says.

Carrier recommends that everyone over 40 reduce their coffee consumption, especially if they work at night.