clipped from www.caffeinedependence.org

Studies have demonstrated that caffeine disrupts sleep. When caffeine is consumed immediately before bedtime or continuously throughout the day, sleep onset may be delayed, total sleep time reduced, normal stages of sleep altered, and the quality of sleep decreased.

Caffeine’s effects on sleep appear to be determined by a variety of factors including dose, the time between caffeine ingestion and attempted sleep, and individual differences in sensitivity and/or tolerance to caffeine.

Caffeine-induced sleep disturbance is greatest among individuals who are not regular caffeine users. Although there is evidence for some tolerance to the sleep disrupting effects of caffeine, complete tolerance may not occur and thus habitual caffeine consumers are still vulnerable to caffeine-induced sleep problems.