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Fifty college students were enrolled in the study for a week. As college students, they tended to be so sleep-deprived that, for most, "it didn't matter how much caffeine they had" – they slept well whenever they finally hit the sack, according to the study researcher Jamie Zeitzer, an assistant professor of psychiatry and behavioral sciences at Stanford University.
However, for the early risers, the more caffeine in their bodies, the more time they spent awake during the night after initially falling asleep. This was not seen in the night owls.
The amount of caffeine in a person at bedtime can vary widely. It is cleared by some within a few hours and in some it can still be present in the system till late at night.
Therefore it's hard to say whether any particular person could avoid the effects of caffeine on sleep by simply steering clear of coffee (or tea) in the afternoon or evening..
The study was published online Feb. 13 in the journal Sleep Medicine.