Charles A. Czeisler, MD, PhD, Chief, Division of Sleep and Circadian Disorders at Brigham and Women's Hospital, explains the impact of sleep on brain function and physical health. During sleep, the brain is flushed of toxins and learning experiences are consolidated. Inadequate sleep can impair physical health including a dampening of the immune response, disruption in hormones that regulate weight, reduction in the effectiveness of insulin metabolism and an increased risk for calcification of the arteries. Dr. Czeisler also describes how artificial light exposure can cause our circadian rhythms to be disrupted resulting in shortened sleep cycles or insomnia.