Negative Effects Of Caffeine Are Stronger On Daytime Sleep Than On Nocturnal Sleep

"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."

"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," explains Dr. Carrier. "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."

Thirty-four moderate caffeine consumers participated in both caffeine (200 mg) and placebo (lactose) conditions in a double-blind crossover design. Seventeen subjects followed their habitual sleep--wake cycle and slept in the laboratory during the night (Night), while 17 subjects were sleep deprived for one night and recovery sleep started in the morning (DayRec). All subjects received a capsule of 100 mg of caffeine (or placebo) 3 hours before bedtime, and the remaining dose 1 hour before bedtime. Compared to placebo, caffeine lengthened sleep latency, increased stage 1, and reduced stage 2 and slow-wave sleep (SWS) in both groups. However, caffeine reduced sleep efficiency more strongly in the DayRec group, and decreased sleep duration and REM sleep only in that group.

The study of Dr. Carrier was supported by scholarships and grants from the Canadian Institutes of Health Research (CIHR), the Fonds de Recherche en Santé du Québec (FRSQ), and the Natural Sciences and Engineering Research Council of Canada (NSERC).