Caffeine has a stronger disruptive effect on daytime, catch-up sleep after a night of sleep deprivation.
Recent studies have suggested that caffeine is a measure of choice to counteract the effects of sleep deprivation on alertness and performance
The results suggest that this recommendation should be made with caution since using caffeine before sleeping at an abnormal circadian phase, such as during night work, might have more adverse consequences on sleep quality than before sleeping at a normal circadian time. Consuming caffeine at night is known to make it more difficult to fall asleep and to worsen sleep quality.
Researchers from the Hospital du Sacre-Coeur de Montreal in Quebec had 34 people consume caffeine or placebo before going to sleep at their normal bedtime. A month later, the same individuals were then deprived of sleep for an entire night, after which they were instructed to fall asleep an hour after their normal wake-up time, again after being given placebo or caffeine. People received 100 mg of caffeine or placebo before their scheduled morning or evening bedtime, and then another 100 mg (or placebo) one hour before. All study participants who took caffeine took longer to fall asleep and spent more time in lighter stage 1 sleep and less time in deeper stage 2 and slow-wave sleep. But people who were given caffeine before daytime sleep took even longer to drop off, and also slept for a shorter total time and had less REM sleep. Caffeine didn’t affect night time sleep duration or REM.
The researchers suggest that the greater daytime effects of caffeine were due to the fact that people were trying to fall asleep at a time when their body clock was telling them to stay awake, even though they were sleep-deprived. Slow-wave sleep is known to last longer during daytime, catch-up sleep, which normally helps override this circadian signal. Given that caffeine reduces slow-wave sleep, it appears to allow the wake-up call to remain strong.