Drinking coffee might be a nice way to round off the evening meal or perk you up in the late afternoon, but it may well disrupt your sleep hours later when you retire for the night, according to a new study reported this week.

Writing in the Journal of Clinical Sleep Medicine, the researchers say this is the first study to look at the effects of a given dose of caffeine taken at different times before a night’s sleep.

Lead author Christopher Drake, associate professor of psychiatry and behavioral neurosciences at Wayne State University in Detroit, MI, says:

“Drinking a big cup of coffee on the way home from work can lead to negative effects on sleep just as if someone were to consume caffeine closer to bedtime.”

Prof. Drake, who is also an investigator at the Henry Ford Sleep Disorders and Research Center, and colleagues say you should probably avoid caffeine after 5 pm if you want a good night’s sleep.

For their study, they recruited 12 healthy normal sleepers and asked them to keep to their normal sleep schedules.

They gave each participant three pills to take each day for 4 days. One pill was to be taken 6 hours before bedtime, the second at 3 hours before, and the third at bedtime.

One of the three pills contained 400 mg of caffeine (equivalent to two or three cups of coffee) and the other two looked identical to the caffeine pill but were only harmless placebos. On one of the days, all three pills were placebos.

Sleep disturbance was measured in two ways: objectively by means of a sleep monitor used at home, and subjectively from diaries kept by the participants.

The results showed that consuming caffeine 3 and even 6 hours before bedtime significantly disrupts sleep – even when consumed 6 hours before bedtime, it reduced objectively measured total sleep time by more than 1 hour.

However, the subjective sleep measures showed that individuals were not aware of any disturbance.

Prof. Drake says:

“People tend to be less likely to detect the disruptive effects of caffeine on sleep when taken in the afternoon.”

Dr. M. Safwan Badr, president of the American Academy of Sleep Medicine, comments that:

Sleep specialists have always suspected that caffeine can disrupt sleep long after it is consumed. This study provides objective evidence supporting the general recommendation that avoiding caffeine in the late afternoon and at night is beneficial for sleep.”

Earlier this year, a study reported in the journal Nature also suggested that light from electronic devices frequently disrupts sleep.