Coffee More Effective in Banishing Daytime Sleep, not Nocturnal One

Despite the fact that coffee's effects against sleepiness were known to be equally potent during nighttime and daytime, a recent study showed that caffeine is more effective in banishing day sleep than bight sleep. Therefore, drinking black coffee to stay up the night when we have to do a night shift or we have to study hard for an exam would not have the powerful expected effects.

The research was carried out by a team of scientists from the Department of Psychology at the University of Montréal and led by Dr. Julie Carrier, director of the Chronobiology Laboratory at the Hôpital du Sacré-Coeur de Montréal.

"We already knew that caffeine has important effects on nocturnal sleep. It increases the time taken to fall asleep, it increases the amount of awakenings, and it decreases the amount of deep sleep. We have shown that these effects of caffeine on sleep are way stronger when taken at night prior to a daytime recovery sleep episode than in the evening before a nocturnal sleep episode," explained Dr. Carrier.

The study was conducted on 34 subjects who drank coffee moderately. They had to take part in two phases of the study - in the first phase, participants had to take 200 mg of coffee and in the second phase they had to take a placebo (lactose). Results of the study showed that sleepiness was considerably reduced in the group of subjects who drank coffee as compared to those in the lactose group. However, the effects of caffeine proved to be stronger against daytime sleep than against nighttime sleep.

The team also pinpointed the fact that if one has to stay up all night long and drinks a large cup of black coffee in the evening, he will be able to keep sleep at distance that night, but his daytime sleep will also be disturbed. "Caffeine makes daytime sleep episodes too shallow to override the signal from the biological clock that tells the body it should be awake at this time of day. We often use coffee and other sources of caffeine during the nighttime to counteract sleepiness generated by sleep deprivation, jet lag, and shift-work. However, this habit may have important effects when you then try to recuperate during daytime," stated Dr. Carrie.