A double espresso-sized dose of caffeine consumed 3 hours before bedtime delayed the normal onset of the melatonin rhythm by about 40 minutes, researchers reported in Science Translational Medicine.
“In addition to increasing daytime exposure to sunlight and reducing evening exposure to electrical light, avoiding evening caffeine may help treat problematic delayed sleep timing,” according to Tina Burke of the University of Colorado Boulder and her associates. The results also could support consuming caffeine in the morning to help recover from jet lag, but further studies would need to test that possibility, the researchers added.
SorJongAng/Thinkstock.com
Caffeine is known to affect circadian rhythms in rats and flies, but its circadian effects in humans were unknown, said the investigators. Their 49-day, double-blinded study included five healthy, normal-weight adults who averaged 24 years of age. For a week before each scheduled laboratory visit, participants slept 8 hours a night as verified with the help of sleep logs, wrist actigraphy, and time-stamped voice mail reminders. In the laboratory, they received caffeine or placebo 3 hours before their normal bedtime and were exposed to either bright or dim (control) light at bedtime (Sci Transl Med. 2015;7:1-9).
Caffeine plus dim light was associated with about a 40-minute longer phase delay than placebo and dim light (P = .011), the investigators reported. Bright light with placebo led to about a 85-minute phase delay (P = .0007), while bright light plus caffeine caused a 105-minute shift (P = .0003). Experiments with cultured human cells also showed that caffeine competitively bound to adenosine receptors, which disrupted signaling of cyclic adenosine monophosphate (cAMP), a key part of the circadian clock, the researchers said.
The study was funded by the National Institutes of Health, the National Center for Advancing Translational Sciences, and the Howard Hughes Medical Institute in collaboration with the University of Colorado. Ms. Burke declared no competing interests.