Coffee has no effect on night owls. Drink coffee to cheer you up.

Coffee refreshes and refreshes, but it can also cause sleeplessness at night. Caffeine affects sleep only for larks who go to bed early and wake up early, but not for night owls, a new study finds.

In the new study, Stanford University researchers asked 50 college students to record their caffeine intake and sleep/wake times for a week. The students wore a device on their wrist that detected their physical activity and recorded their waking state during sleep at night. The researchers also measured caffeine levels in the saliva of each student over the course of a week. It turns out that larks who consume caffeine during the day are more likely to wake up at night and interrupt sleep. The researchers say the new study is the first to find an association between caffeine intake and "time patterns"-the time of day when people are most alert or active.

Dr. Jamie Zeitzler, associate professor of psychiatry and behavioral sciences who led the new study, said most college students are sleep-deprived and fall asleep no matter how much coffee they drink. But for Larks, the more caffeine they ingested, the longer it took for wakefulness to occur during sleep. This is not true of night owls. The next step in the study will be to conduct the same study in non-college students to determine whether caffeine interferes with sleep only in larks is universal.

Dr. Zeitzler says everyone metabolizes caffeine differently. Some people metabolize caffeine in a matter of hours, while others drink coffee at noon and still have caffeine in their bodies at night. Therefore, if you are a lark, it is best not to consume too much caffeine such as coffee or tea after noon.