Caffeine can help improve alertness and performance in those with restricted sleep patterns but the effects wane after the second consecutive day with 5 hours of time in bed (TIB), researchers presented at SLEEP 2016, the 30th Anniversary Meeting of the Associated Professional Sleep Societies.

The findings come from a new double blind, placebo-controlled study that tested the effects of 200mg of caffeine or placebo, twice daily (0800 and 1200hrs) for 5 days. In total, 48 healthy individuals took part and they were restricted to 5 hours TIB for 5 consecutive days. Before the 5 days of sleep restriction, the participants had 5 nights of 10-hour TIB sleep satiation, and then 1 full day of baseline testing. During the final 3 days of the study, the participants were allowed recovery sleep (8hrs TIB per night). 

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The results showed that the caffeine group performed a 10-minute Psychomotor Vigilance Task (PVT) significantly better than the placebo group. However, the improved performance was only evident for the first 2 days of sleep restriction and results decreased to resemble that of the placebo group’s performance for the final 3 days.

Similarly, sleep latency and improvements in happiness were significantly greater in the caffeine group but this difference was only noted for the first 2 days. In the final days of sleep restriction, the caffeine group reported being more annoyed than the control group. Moods and alertness were measured via the Profile of Mood States (POMS) test, and the Stanford Sleepiness Scale (SSS). Additionally, a modified Maintenance of Wakefulness Test (mMWT) was administered approximately every 4 hours during wake time.

The authors suggest that more research is needed to deduce whether the reason for the waning effect of caffeine was due to “mounting sleep debt and high cognitive load.” 

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