It's common knowledge that, generally speaking, caffeine disrupts sleep.
For those who can't do without their coffee, it has always been recommended to drink in moderation and stop consuming coffee after 3pm to minimize the negative effects on sleep.
This is still good advice, but researchers from the Henry Ford Hospital and Wayne State College of Medicine in Detroit have found that caffeine still disrupts sleep even if you stop consuming it early in the day.
This latest study involved a small group of healthy adults without sleep issues. All regularly slept for between 6.5 and 9 hours each night and fell asleep within half an hour of getting into bed.
Participants were asked to take 3 pills over the course of a day, every other day.
One pill was taken 6 hours before their normal bedtime, one pill was taken 3 hours before their normal bedtime and one pill was taken at bedtime.
On one of the the study days, every single pill was a placebo. On the other days, one of the pills contained caffeine (400mg). The time it was taken (6, 3, or 0 hours before bedtime) was determined at random.
A sleep monitor combined with individual sleep diaries helped researchers determine the effect the caffeine pills had on each participant's sleep.
Researchers found that caffeine significantly reduced total sleep time regardless of the time it was consumed.
Total sleep time was reduced by between 1.1 and 1.2 hours, regardless of when the caffeine was consumed.
Other measures of sleep quality were also affected.
Compared to the placebo, caffeine taken 6 hours before bed increased the amount of time taken to fall asleep by 24 minutes. Caffeine taken 3 hours before bed increased the amount of time taken to fall asleep by 17 minutes, and caffeine taken at bedtime increased the amount of time taken to fall asleep by 22 minutes.
Similarly, both stage 1 and stage 2 sleep were significantly reduced regardless of when the caffeine was consumed.
In conclusion, 400mg of caffeine (equivalent to a large, store-bought coffee) can significantly disrupt sleep regardless of when it is consumed.
Source: American Academy of Sleep Medicine (PDF)
Last updated: January 8, 2014