Q. Are there any foods I can eat to help me fall asleep? Sleeping pills and herbal products leave me feeling groggy in the morning.

A. A high-carbohydrate snack one to four hours before bedtime could help. Although carbohydrates give us energy during the day, they also can help us fall asleep at night.

This has a lot to do with a substance produced in our brains called serotonin. Serotonin is the body's own natural sleep inducer. Your brain needs tryptophan, an amino acid to form serotonin.

Carbohydrates boost serotonin levels in the brain. After eating a meal or snack that contains mostly carbohydrates, blood insulin levels rise. Insulin reduces blood levels of most amino acids except for tryptophan. This gives tryptophan easy access to the brain. Once in the brain, tryptophan creates serotonin. This process works only when the last meal or snack you eat is almost exclusively composed of carbohydrate.

You may be wondering why tryptophan-containing protein foods such as turkey, milk or salmon don't have this effect. On Thanksgiving, it is the large meal that makes you sleepy, not the tryptophan in turkey.

The reason is that a barrier exists between our blood and our brain. It acts like a locked door allowing only a few select substances to enter the brain. Tryptophan must compete with the other amino acids to enter the brain. Since the amount of tryptophan is smaller than these other amino acids in protein foods, it doesn't make it in.

This connection between carbohydrates, serotonin and sleep was initially discovered in the 1980s by Judith and Richard Wurtman of the Massachusetts Institute of Technology. They found that an evening snack that was mostly carbohydrate such as English muffin with jelly or graham crackers helped many people fall asleep.

A recent study by Australian researchers suggests that carbohydrates with a high glycemic index may be best for inducing sleep. High-glycemic foods cause a large spike in blood sugar after being eaten. This leads to a higher blood insulin levels production, helping tryptophan to enter the brain. Examples of high-glycemic foods include jasmine rice, white rice, pasta and potatoes.

Stress, physical activity, medications, caffeine, and alcohol intake can also influence our ability to sleep. Dealing with the cause is the best strategy; however, a few soda crackers or toast (plain or with jelly) before bed is worth a try.

Amy Tousman is a registered dietitian and certified diabetes educator with Nutrition Unlimited in Kailua. Hawai'i experts in traditional medicine, naturopathic medicine and diet take turns writing the Prescriptions column. Send your questions to: Prescriptions, Island Life, The Advertiser, P.O. Box 3110, Honolulu, HI 96802; islandlife@honoluluadvertiser.com; or fax 535-8170. This column is not intended to provide medical advice.