Word to the wise to your patients who need their caffeine fix: 5 pm should be the cutoff for that last cup of coffee or energy drink. A new study, published in the Journal of Clinical Sleep Medicine‘s Nov 15 issue, shows that caffeine consumption even six hours before bedtime can have significant, disruptive effects on sleep. “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,” says lead author Christopher Drake, PhD, investigator at the Henry Ford Sleep Disorders and Research Center and associate professor of psychiatry and behavioral neurosciences at Wayne State University in Detroit, Mich, in a release. “People tend to be less likely to detect the disruptive effects of caffeine on sleep when taken in the afternoon,” Drake says.

Drake and his research team studied 12 healthy normal sleepers, as determined by a physical examination and clinical interview. Participants were instructed to maintain their normal sleep schedules. They were given three pills a day for four days, taking one pill at six, three, and zero hours prior to scheduled bedtime. One of the pills contained 400 mg of caffeine (about 2-3 cups of coffee), and the other two were a placebo. On one of the four days, all three pills were a placebo. Sleep disturbance was measured subjectively with a standard sleep diary and objectively using an in-home sleep monitor. Results show that the caffeine taken at all three times reduced total sleep time dramatically. However, subjective reports suggest that participants were unaware of this sleep disturbance.