When foods with a high glycemic index are consumed, the carbohydrates in the food are more easily digested than low glycemic index foods. Hence, more glucose is available for absorption. It should not be misunderstood that glucose is absorbed more rapidly because, once formed, glucose is absorbed at the same rate. It is only available in higher amounts due to the ease of digestion of high glycemic index foods. In individuals with normal carbohydrate metabolism, insulin levels rise concordantly to drive glucose into the body's tissues and maintain blood glucose levels in the normal range. Insulin stimulates the uptake of valine, leucine, and isoleucine into skeletal muscle, but not uptake of tryptophan. This lowers the ratio of these branched-chain amino acids in the bloodstream relative to tryptophan (an aromatic amino acid), making tryptophan preferentially available to the large neutral amino acid transporter at the blood–brain barrier. Uptake of tryptophan by the brain thus increases. In the brain, tryptophan is converted to serotonin, which is then converted to melatonin. Increased brain serotonin and melatonin levels result in sleepiness.